UNIVERSITY  OF  CALIFORNIA 

AT   LOS  ANGELES 


AN   OUTLINE    OF    THE 
THEORY  OF  ORGANIC  EVOLUTION 


1249     16F 


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Neritina  virginea,  variety7  minor. 


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.Jnsgiavib  lavawori  .all'jria  aaaril  lo  owl  ynB  naav/lad  aaiiaa  gnibBigialni 


HiE    MAt 


PLATE  i.  —  Frontispiece.  Variation  in  color  and  in  color  pattern  in  Neritina  virginea,  variety 
minor.  Magnified  two  diameters. 

Color  pattern  : 

i,  marked  with  a  few  heavy  lines.  From  i  to  6  these  major  lines  become  broken  up  into 
small  V-shaped  loops.  In  the  shells,  a,  accessory  minor  lines  are  added.  In  the  shells,  b,  these 
are  more  numerous. 

Series  9  to  14  shows  diversity  in  the  pattern  near  the  apex  of  the  coil :  9  has  a  few  very  slightly 
larger  white  dots  near  the  coil;  10  has  larger  dots  here;  n  has  them  very  large;  in  12  they  have 
united  to  form  a  continuous  white  band;  in  13  and  14  this  band  is  wider. 

Series  15  to  24  shows  diversity  in  the  character  of  the  equatorial  light  band.  In  15  and  16 
only  the  minor  lines  are  interrupted  or  faint  along  the  equator  of  the  shell.  In  17  the  major  lines 
also  are  interrupted.  In  18  the  band  is  almost  clear  white.  19  and  20  show  narrower  bands.  In 
21,  23,  and  24  the  equatorial  band  is  shown  by  a  difference  of  color  in  or  under  the  pattern.  In  22 
the  equatorial  line  is  faintly  indicated  in  the  pattern  itself,  being  bordered  above  and  below  by 
large,  heavy,  black  loops. 

Color  shade : 

The  colored  .lines  are  black  in  i,  3,  5,  5  b,  6,  6  a,  19,  ap,  and  22;  purplish  in  6b  and  n  ;  red  in 
7  a  ;  gray  in  23 ;  black  and  red  in  2 ;  the  major  lines  are  black  and  the  minor  linos  red  in  i  a,  3, 
3  a,  and  5  a;  the  major  lines  are  black  and  the  minor  lines  purple  in  16  and  17. 

These  are  a  few  shells  selected  from  a  large  double-handful  scooped  up  from  the  sand  beach 
of  the  '*,Salt  Pond,"  near  Port  Henderson,  Jamaica,  W.I.  The  shells  were  so  numerous  as  to 
completely  cover  the  beach  for  rods  at  the  water's  edge.  Sixty- eight  quite  distinct  varieties  in 
color  or  color  pattern  were  found  in  this  one  pint  of  shells.  It  is  possible  to  find  a  completely 
intergrading  series  between  any  two  of  these  shells,  however  divergent. 


AN  OUTLINE  OF  THE  THEORY 


OF 


ORGANIC    EVOLUTION 


WITH  A  DESCRIPTION   OF  SOME  OF  THE 
PHENOMENA  WHICH    IT   EXPLAINS 


BY 


MAYNARD    M.    METCALF,   PH.D. 

PROFESSOR  OF  ZOOLOGY,  OBERLIN  COLLEGE 


THIRD    EDITION 
REVISED 


f|0rfe 
THE    MACMILLAN   COMPANY 

LONDON:   MACMILLAN  &  CO..  LTD. 


All  rights  reserved 

&4I47 


COPYRIGHT,  1904, 
BY  THE  MACMILLAN   COMPANY. 


Set  up  and  electrotyped.      Published   October,  1904. 
Second  edition  July,  1906.     Third  edition,  September,  1911. 


Norivocd  Press 

.  S.  Gushing  &  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


Eo 


WHO   LOVED   AND   HELD   FRIENDLY   INTERCOURSE 
WITH   NATURE 


Tom  to  Mother  Carey  :  "  I  heard,  ma'am,  that  you  were 
always  making  new  beasts  out  of  old." 

Mother  Carey :  "  So  people  fancy.  But  I  am  not  going 
to  trouble  myself  to  make  things,  my  little  dear.  I  sit 
here  and  make  them  make  themselves." 

—  CHARLES  KINGSLEY'S  "  THE  WATER  BABIES." 


PREFACE    TO    FIRST    EDITION 

THE  lectures  out  of  which  this  book  has  grown  were 
written  for  the  author's  students  at  the  Woman's  College  of 
Baltimore,  and  for  others  in  the  college  not  familiar  with 
biology  who  had  expressed  a  desire -to  attend  such  a  course 
of  lectures.  The  book  is,  therefore,  not  intended  for  biolo- 
gists, but  rather  for  those  who  would  like  a  brief  introductory 
outline  of  this  important  phase  of  biological  theory. 

It  has  been  the  author's  endeavor  to  avoid  technicality 
so  far  as  possible,  and  present  the  subject  in  a  way  that  will 
be  intelligible  to  those  unfamiliar  with  biological  phenomena. 
The  subject,  however,  is  somewhat  intricate,  and  cannot  be 
presented  in  so  simple  a  manner  as  to  require  no  thought 
on  the  reader's  part ;  but  it  is  hoped  that  the  interest  of  the 
subject  will  make  the  few  hours  spent  in  the  perusal  of  this 
book  a  pleasure  rather  than  a  burden. 

In  many  instances  matter  that  might  have  been  elabo- 
rated in  the  text  has  been  treated  in  the  pictures,  which,  with 
their  appended  explanations,  form  an  essential  part  of  the 
presentation  of  the  subject.  This  method  of  treatment  has 
been  chosen  both  for  the  sake  of  the  greater  vividness  thus 
secured  and  because  it  enables  the  book  to  be  reduced  to  the 
limits  desired.  Many  of  the  illustrations  have  been  obtained 
from  books  with  which  the  reader  may  wish  later  to  become 
familiar. 

In  his  lectures  upon  evolution  the  author  made  no 
attempt  to  avoid  following  the  manner  of  presentation  or 
even  the  phraseology  of  prominent  writers  upon  the  subject, 


Vlll  PREFACE 

and  for  this  book  little  claim  to  originality  can  be  made. 
The  author  has  attempted  to  present  the  subject  in  the  way 
that  seemed  simplest  and  most  natural  to  him,  realizing  that 
in  so  doing  he  would  almost  necessarily  follow  in  large  meas- 
ure the  authors  who  have  influenced  his  thinking  upon  the 
subject.  He  is  especially  indebted  to  his  former  instructor, 
Professor  W.  K.  Brooks,  than  whom  there  is  no  clearer 
thinker  in  the  field  of  evolution. 

There  are  a  number  of  very  valuable  books  which  treat 
of  the  evolution  theory.  Most  prominent  among  these  are 
the  writings  of  Darwin  and  Wallace,  and  Romanes'  Dar- 
win and  After  Darwin..  The  author  does  not  intend  that 
this  volume  shall  be  accepted  by  any  reader  as  a  substitute 
for  those  more  important  books,  but  rather  that  it  shall  serve 
as  an  introduction  to  the  subject,  giving  a  comprehensive 
outline  of  the  theory,  with  just  sufficient  illustration  to  invite 
the  reader  to  seek  fuller  knowledge  of  the  great  number  of 
most  interesting  phenomena  which  are  related  to  the  theory. 
At  the  end  of  this  book  will  be  found  a  list  of  a  few  of  the 
more  important  volumes  treating  of  the  theory  of  evolution 
and  the  phenomena  which  it  explains. 

In  the  preparation  of  this  book,  especially  in  securing  or 
preparing  the  pictures,  the  author  has  received  much  assist- 
ance and  many  courtesies.  It  is  a  pleasure  to  him  to 
acknowledge  his  indebtedness  : 

For  the  gift  or  loan  of  photographs  or  material  for  illus- 
tration, to  the  authorities  of  the  United  States  National 
Museum,  the  American  Museum  of  Natural  History,  the 
United  States  Department  of  Agriculture,  the  United  States 
Fish  Commission,  to  A.  Radcliffe  Dugmore,  Rev.  Dr.  John 
T.  Gulick,  Mr.  C.  L.  Allen,  and  especially  to  his  friend, 
Horace  W.  Britcher,  whose  untimely  death  has  removed  one 
of  our  keenest  students  of  living  spiders ; 

For  assistance  in  the  identification  or  in  the  preparation 
of  material  for  illustrations,  to  Dr.  Harrison  G.  Dyar, 


PREFACE  ix 

Dr.  L.  O.  Howard,  Dr.  F.  H.  Chittcnden,  Dr.  Charles  W. 
Richmond,  Miss  Mary  J.  Rathbun,  Mr.  Nathan  Banks,  and 
Professor  L.  H.  Merrill ; 

For  generously  giving  permission  to  copy  certain  figures, 
to  the  Open  Court  Publishing  Company,  Macmillan  &  Com- 
pany, D.  Appleton  &  Company,  Edward  Arnold,  Bradlee 
Whidden,  Swan  Sonnenschein  &  Company,  Smith,  Elder  & 
Company,  Charles  Scribner's  Sons,  E.  P.  Dutton  Company, 
The  Crowell  Publishing  Company,  to  Professor  August 
Weismann,  Professor  E.  B.  Poulton,  Dr.  and  Mrs.  G.  W. 
Peckham,  Rev.  Dr.  H.  C.  McCook,  Mr.  A.  R.  Dugmore,  Dr. 
F.  M.  Chapman,  President  D.  S.  Jordan,  Professor  Vernon 
L.  Kellogg,  and  Hon.  Addison  Brown ; 

For  kindly  selling  the  right  to  use  certain  figures,  to 
Doubleday,  Page  &  Company,  A.  &  C.  Black,  the  Autotype 
Company,  and  A.  G.  Wallihan ; 

For  assistance  in  revising  certain  paragraphs,  to  Dr.  C. 
Hart  Merriam  and  Professor  W.  B.  Clark ; 

For  assistance  in  revising  proof  of  all  of  the  illustrations, 
to  Mr.  Max  Broedel. 


PREFACE   TO   SECOND   EDITION 

IN  this  second  edition  a  few  slight  modifications  of  the 
text  have  been  made  for  the  sake  of  greater  clearness,  several 
inadvertent  errors  have  been  rectified,  and  mistakes  have 
been  corrected  in  two  of  the  plates  (76  and  77)  which 
were  borrowed  without  sufficient  scrutiny.  The  author  de- 
sires to  acknowledge  with  most  cordial  appreciation  the 
kindness  of  Professor  E.  B.  Poulton,  who  pointed  out  the 
errors  in  these  plates.  In  a  few  instances  proper  credit  was 
not  given  for  borrowed  figures.  These  omissions  have  now 
been  supplied.  Also  a  few  titles  have  been  added  to  the  list 
of  books  in  the  Appendix. 

In  the  first  edition  of  this  book,  the  author  suggested 
very  briefly  that  there  might  be  inherent  tendencies  in 
organisms,  leading  them  to  evolve  in  certain  directions  rather 
than  in  others.  In  Appendix  I  to  this  edition,  some  further 
evidence  for  this  view  has  been  given,  and  Weismann's  sug- 
gestion as  to  a  possible  explanation  of  these  tendencies  has 
been  briefly  treated.  It  has  seemed  best,  also,  in  Appendix  I, 
to  discuss  a  little  further  the  influence  of  individual  plasticity 
upon  evolution. 

For  all  the  kindly  comments,  and  especially  for  criticism, 
upon  this  book,  the  author  feels  very  grateful.  He  was  at 
first  doubtful  if  the  published  lectures  would  be  useful,  and  it 
is  a  satisfaction  to  know  that  they  have  found  a  place  and  are 
apparently  proving  helpful. 


PREFACE    TO    THIRD    EDITION 

THE  call  for  a  third  edition  of  this  volume  gives  opportu- 
nity for  a  second  revision.  The  treatment  of  variation  has 
been  modified  to  accord  with  recent  studies  of  mutation, 
orthogenesis  has  been  more  emphasized,  Reighard's  work 
upon  immunity  coloration  has  been  mentioned,  and  minor 
changes  have  been  made  throughout.  A  few  titles  have 
been  added  to  the  list  of  books  upon  evolution. 

In  recent  discussions  of  evolution  great  emphasis  has 
been  laid  upon  mutation,  as  is  but  natural,  since  this  is  the 
newest  field  of  discovery.  Yet,  viewing  evolution  as  a  whole 
and  somewhat  in  the  light  of  the  historical  development  of 
our  knowledge,  such  extreme  emphasis  is  disproportionate. 
In  this  volume  therefore  mutation  receives  less  extended 
treatment  than  in  most  recent  books  upon  evolution.  Adap- 
tation of  organisms  to  their  environment  is  recognized  as  the 
most  salient  fact  in  that  evolution  which  has  taken  place  and 
natural  selection  is  emphasized  as  giving  us  the  key  to  this 
adaptation. 

Little  discussion  of  eugenics  is  included  —  partly  because 
this  science  deals  as  yet  chiefly  with  social  progress  and  but 
little  with  true  evolution,  because  it  attempts  to  bring  man- 
kind nearer  to  its  present  best  rather  than  to  advance  thij 
best  to  a  still  better  type;  but  such  discussion  is  omitted 
chiefly  because  it  demands  fuller  treatment  than  is  possible 
within  the  purpose  of  this  volume. 


TABLE    OF   CONTENTS 


INTRODUCTION 


PART   I 

THE  THEORY  OF  ORGANIC  EVOLUTION         .... 
Natural  Selection      ........ 

Heredity   .         .         .         .         .         ..       . 

Variation  (including  mutation)          .         . 

The  Struggle  for  Existence    '    . '       .         . 

General  Principles  in  the  Operation  of  Natural  Selection 

Artificial  Selection     .         .        •.         ...      .  . , 

Objections  to  Natural  Selection  as  a  Factor  in  Evolution 
Orthogenesis    .         .         .         .         .         .         ... 

Sexual  Selection       .         .         .         .         .         .         .         . 

Objections  to  the  Theory  of  Sexual  Selection    . 
Segregation      .         .  •      .         .         .         . 
The  Inheritance  of  Parental  Modifications 
Summary  of  Part  I  .         .         .         .         .         .      '  .         . 


3-86 

3-5° 
3-12 
7-12 

12-21 
2I-3O 
30-33 

33-49 
49-50 
50-63 

59-63 
63-70 
70-85 
85-86 


PART   II 

THE  PHENOMENA  EXPLAINED  BY  THE  THEORY- 
Comparative  Anatomy 

Classification     . 

Homology          .         . 

Vestigial  Structures  .... 
Embryology     .         .      .  .        x.         » 
Paleontology    .         .         .         .  .  :   .         , 
Geographical  Distribution 
Color  in  Animals      ..... 

Protective  Coloration  and  Resemblances 

Aggressive  Coloration  and  Resemblances 


87-166 
90-98 
90-94 

94-95 
95-98 
98-105 
105-113 
113-118 
118-154 
119-127 
127-129 


xiv  TABLE    OF  CONTENTS 

PAGES 

Alluring  Coloration  and  Resemblances      .....  129-131 

Warning  Coloration           .         .         .         .         .         .         .         .  132-136 

Convergence  in  Warning  Coloration  .         .         .         .         .  .136 

Immunity  Coloration          .         .         .         ...         .         .  .137 

Mimicry    .         .         .         .         .         .         ...'-.'..         .  137-149 

Protective  Mimicry     .         .         ...         .    *    .         .         .  137-148 

Aggressive  Mimicry    ...*....  148-149 

Signals  and  Recognition  Marks          .         .                  .         .         .  149-150 

Confusing  Coloration         .         .                  .         .         .         .         .  150-152 

Sexual  Coloration      .         .         ...         .         .         .         .  152-153 

Summary  of  the  Treatment  of  Color  in  Animals         .         .         .  .154 

Color  in  Plants         ...         .         .         .         .         .         .         .  154-166 

MAN  IN  RELATION  TO  EVOLUTION 166-186 

GENERAL  CONSIDERATIONS 187-191 

APPENDIX  I.  —  Trends  in  evolution,  germinal  selection,  organic  selection  193-200 

APPENDIX  II.  —  A  few  books  which  treat  of  organic  evolution  and  phe- 
nomena of  special  adaptation   .....  201-203 

INDEX 205-217 


LIST    OF    ILLUSTRATIONS 

(!N  THE  ORDER  OF  THEIR  INSERTION) 

PAGE 

Variation  in   color  and  in  color  pattern  in  Neritina   virginea, 


variety  minor.     (In  color)      .....         Frontispiece 

3. 

I. 

Goose-barnacle    .......... 

4 

3. 

2. 

Gerarde's  figure  of  "  Barnacles  producing  geise  " 

5 

:E 

2. 

Variation  in  Trillium  grandiflorntn      ....  Following 

6 

;E 

3- 

Varieties  of  Paludestrina  protea  ....." 

8 

3. 

3- 

"  Bag-worm,"  Thyroidopteryx  ephemeriforinis      .         .         . 

23 

G. 

4- 

Honey-bees          .......... 

24 

TE 

4- 

Varieties  of  horses        .         .          .         .''.-.          .Following 

3° 

TE 

4,  a 

.     The  wild  cabbage  {Brassica  oleracea)      .         .         .  Following 

30 

CES 

5-7 

,     Varieties  of  cabbage  :  Savoy  cabbage,  kale,  broccoli,  Brussels 

sprouts,  cauliflower,  Swedish  turnip,  and  kohlrabi    .  Following 

3° 

TE 

8. 

Varieties  of  cabbage,  etc.,  as  figured  in  Gerarde's  Herball,  six- 

teenth century         .         .         ...         .         .  Following 

3° 

TE 

9- 

Varieties  of  turnips       .         .                  .         ...         " 

3° 

Pi 

TE 

10. 

Varieties  of  dahlias      ....         .         .         .         " 

3° 

1  i., 

TE 

n. 

•'  Cactus  "  type  of  dahlia       .         .         .         ...         " 

3° 

J 

G. 

5- 

Skull  of  Polish  fowl     ...',.        .         .         . 

32 

PL. 

TES 

12-15.     Varieties  of  domestic  chickens.     (In  color)    .         .Following 

32 

PL 

TE 

16. 

A.  Jungle   fowl  (Gallus  bankiva).     B.     The   evolution   of  the 

game  cock       .         .         .         .         .         .                  .  Following 

32 

PL, 

TE 

i?- 

Japanese  long-tailed  cocks   .         .         ;         .                  .         " 

32 

PL, 

TE 

18. 

A,  "Frizzled    fowls."     B.  Head   of   Breda    cock.     C.  Head   of 

salmon  faverolle      .......  Following 

32 

PL, 

TE 

19. 

A.  Feather    from   a   "silky   fowl."      B.  Leg    of   Cochin    cock. 

C.  "  Cochin  "  bantams    ......  Following 

32 

PL 

TE 

20. 

Varieties  of  domestic  pigeons       ....." 

32 

IG. 

6. 

Rock  pigeon  (Coltemba  livia)       .         .         .         ... 

33 

PL. 

TE 

21. 

Skeletons  of  various  unicellular  animals  and  plants       .  Following 

34 

Pi  , 

TE 

22. 

Male  and  female  bobolink  {Dolichonyx  oryzivorus)       .         " 

52 

PL. 

TE 

23- 

Ruffed  grouse  (Bonasa  umbellus),  male,  female,  and  young     •' 

52 

PL, 

TE 

24. 

A.  Male  and  female  argus  pheasant.     B.  Male  and  female  lyre 

bird        .         .         .         .         .         ...         .  Following 

52 

PL, 

TE 

25. 

A.  Male  and  female  Nesocentor  milo.     B.  Male  and  female  pigeon 

(Phlogcenas  jobiensis)       .         ...         .         .  Following 

52 

' 

TE 

26. 

Male  and  female  humming-birds           .         .         .                  " 

52 

.TE 

27. 

Turkey  cock  '•  strutting  "      .         .         .         .         ,  '      .         " 

52 

PL, 

,TE 

28. 

Courting  attitudes  in  hunting  spiders   ...         .         " 

54 

XV 

xvi 


LIST  OF  ILLUSTRATIONS 


PLATE  29.     A.  Male  and  female  seventeen-year  cicada.     B.  Staghorn  beetle, 
males  and  female    .         .         .         .         .         .         .  Following 

PLATE  30.     Male  and  female  Hercules  beetle  « 

FIG.     7.     Heads  of  male  and  female  beetles 

PLATE  31.     Male,  female,  and  larva  of  Chauliodes  cornutrts     .         .Following 
PLATE  32.     Male  and  female  fish  :     A.   Callionymus  lyra.     B.  Xiphophorus 

heller ii .         .         .  Following 

PLATE  33.     A.  Male  and  female  dragon-fly  (Calopteryxmaculata).     B.  Male, 
female,  and  larva  of  crested  newt  (Triton  cristatus)  Following 
PLATE  34.     Males  and  females  of  different  species  of  lizards    .         .         " 

FIG.     8.     Secondary  sexual  characters  in  copepods      .         .         . 

FIG.     9.     Locusts  from  the  Galapagos  Islands     ...... 

FIG.   10.     Map  of  Oahu,  Hawaiian  Islands  .         •         .         .         . 

FIG.   n.      Viola  cucullata    .         .    ..''•.         .         .    -    . 

FIG.   12.     Viola  rostrata      .         .         .      "..         .         .     *   . 

FIG.   13.     Solea  concohr     ..         .         .         .         .         .         . 

FIG.   14.     Skeletons  of  the  fore  limbs  of  various  vertebrates          .         . 

FIG.   15.     Vestigial  bones  of  the  hind  limbs  in  a  boa  constrictor  . 

FIG.   16.     Skeleton  of  Greenland  whale        .         .         .         .         .         •      .  • 

PLATE  35.     Apteryx  australis         ......         .  Follmving 

PLATE  36.     Eyes  of  various  vertebrates,  showing  the  nictitating  membrane 

Following 

PLATE  37.     Hair  tracts  on  the  arms  and  hands  of  a  man  and  a  male  chim- 
panzee   .........  Following 

FIG.   17.     Muscles  of  the  human  ear    .         .         .         .         . 

FIG.   18.     Three  fishes,  showing  stages  in  the  loss  of  eyes  and  color    . 

FIG.   19.     Stages  in  the  development  of  the  pond  snail  {Lymneens) 

FIG.  20.     Tadpole  of  salamander         .         .         ... 

PLATE  38.     Embryos  of  various  vertebrates    .         .         .         .         .Following 

PLATE  39.     American  lobster          .         .         .         ...         .  Following 

PLATE  40.     A.  Central  nervous  system  of  crawfish.    B.  "  Blue  crabs  "       " 
PLATE  41.     A.  "  Mysis  stage  "  in  the  development  of  the  American  lobster. 
B.  Mysis  stenolepis.     C,  Leg  of  Mysis  stenolepis    .  Following 

FIG.  21.     Three  stages  in  the  development  of  a  crab    .         .         .         . 

FIG.  22.     Hydra.     A  diagrammatic  longitudinal  section       .... 

FIG  .  23 .     Gastrula  of  a  coral  polyp  (Monaxenia  darwitiii) 
PLATE  42.     Longitudinal  sections  of  gastrulae  of  :  A.  frog,  young.     B.  frog, 
older.      C.  chick     .         .         .  .         .         .  Following 

FIG.  24.     Longitudinal  sections  of  gastrulae  of  various  animals    . 
PLATE  43.     Antlers  of  a  stag,  showing  the  addition  of  new  branches  in  suc- 
cessive years  ........  Following 

FIG.  25.     Fossil  deer  antlers        .         .         .         .         .         .         • 

FIG.  26.     Successive  forms  of  Paludina  from  the  tertiary  deposits  of  Slavonia 

PLATE  44.     Archczopteryx  lithographica          .....  Following 

PLATE  45.     Fossil    skeletons   of:    A.  Hesperornis   regalis.     B.  Ichthyornis 

victor.     C.  Phrodactylus  spectabilis         ..       .         .Following 

FIG.   27.     Skeleton  of  a  crow        ......... 


54 
54 
55 
54 

56 

56 
56 
60 

65 
67 
90 

91 
92 

94 
96 
96 
96 

96 

96 
97 
97 
99 
100 

IOO 
IOO 


IOO 
102 

103 
104 

104 
105 

108 

109 

I  IO 
I  IO 

110 
112 


LIST  OF  ILLUSTRATIONS 


xvn 


PAGE 

PLATE  46.     Fossil  skeleton  of  Phenacodus  primavus      .         .         .Following  112 
PLATE  47.     Changes  in  foot-structure  and  teeth  in  fossil  and  recent  species  of 

the  horse  family     ...        .     ;    .     -    .         .         .         .  Following  1 1 2 

FIG.  28.     Map  of  southeastern  Asia,  the  East  Indies,  and  Australia     .         -117 

PLATE  48.     A.  Bluefish.     B.  Sand  flounder  .         .         .         .         .Following  120 

PLATE  49.     A.  Field  sparrows.     B.  Quail     .         ...         .        "  120 

PLATE  50.     Woodcock  on  nest       .         .         .        ...        .        "  120 

PLATE  51.     A.  Nighthawk.     B.  Humming-bird's  nest  ....        "  120 

FIG.  29.     A  straw-colored  spider  (  Tetragnatha  grallator)  in  its  accustomed 

position  on  a  blade  of  dead  grass    .         ..       .         .         ;         .  122 

PLATE  52.     Tree  lizards  on  oak  bark      .         .         .         .                  .Following  122 
PLATE  53.     Protectively     colored      mammals.        A.    "Cotton-tail"    rabbit. 

B.  Thirteen-striped  spermophile     .         '.         .         .Following  122 
PLATE  54.     A.  Cony  (Otochond)  among  rocks.     B.  Moth  on  bark         "  122 
PLATE  55.     Protectively  colored  woods-moths.     (In  color)     .         .         "  122 
PLATE  56.     Protectively  colored  caterpillars.     (In  color)         .         .         "  122 
PLATE  57.     Snow    grouse    in    winter,    spring,    summer,   and    fall    plumage 

Following  122 
PLATE  58.     Grass   porgy,   showing   changes    in    color    occurring    in    a    few 

moments         .         .         .         .         .         .    '     .         .Following  122 

PLATE  59.     Color  adaptation  in  pupae  of  Pieris  rapes  and  Vanessa  urtica. 

(In  color)       .         .         ...         .         .         .         .Following  122 

FIG.  30.     Twig-like  caterpillar  of  the  moth  Selenia  tetralunaria           .         .  124 
PLATE  60.     Caterpillar  of  the  moth  Catocala  amatrix,  on  a  poplar  twig  . 

Following  1 24 

PLATE  61.     A.  "Walking  sticks"  on  a  twig.     B.  "  Moss  insect ".         "  124 
PLATE  62.     Leaf  insects.     A.  Locust  (Cycloptera  ).     B.  Mantis  (P/iyllimn) 

C.  Longicorn  beetle  (Mormolyce)  .         .                  .  Following  \  24 
PLATE  63.     Logoa  opercularis  and  L.  crispata,  adults,  larva;,  and  cocoons 

Following  1 24 

FIG.  31.     A  crab  (Cryptolithodes  sitchensis)  which  resembles  a  pebble         .  126 
FIG.  32.     A  "  sea-horse  *'  (Hippocampus)   .         .         .         .         .         .         .126 

PLATE  64.     Spiders  whose  color  and  shape  render  them  difficult  to  see  . 

Following  126 
PLATE  65.     Sargassum  fish  (Pterophryne  histrio)  in  a  tuft  of  floating  seaweed. 

(In  color)        ....        .         ..        ..        ..        .         .Following  126 

FIG.  33.     Tree-frogs  whose  backs  resemble  oak  leaves  in  color  and  color 

pattern  .         .         .         .         .         .        ..         .         .         .         .127 

FIG.  34.     Polar  bear   .         .         ...         .         .         .         .         .         .         .128 

PLATE  66.     A.  Tree-frog  on  bark.     B.  Common  toads           .         .  Following  128 

PLATE  67.     Weasels  in  winter  and  in  summer  pelage      •  '-.  ,   f        •         "  128 

PLATE  68.     A.  Tiger.     B.  Jaguar         .        .        .        ...        "  128 

FIG.  35.     Arctic  fox,  in  winter  and  in  summer  pelage  ,         .         .         .129 

FIG.  36.     A  mantis  (Hymenopus)  which  resembles  an  orchid  blossom         .  130 
PLATE  69.     Warning  form  and  coloration  in :  A.  Two  bugs  (Prionotus  and 
EuchistHs).     B.  Lady-beetles.     C.  Colorado  potato  beetle     . 

Following  132 


XV111 


LIST  OF  ILLUSTRATIONS 


PLATE  70.     Warning  coloration  and  mimicry  in  moths.    (In  color)     Following 
PLATE  71.     Inedible  caterpillars,  showing  warning  coloration.     (In  color) 

Following 

PLATE  72.     A.  Gila  monster  (Heloderma).     B.  Species  of  skunks  belonging 
to  the  sub-genus  Chincha         .         .         .         .         .  Following 
FIG.  37.     Salamander  (Salamandra  maculosd) 
PLATE  73.     Inedible  curculios  and  lady-beetles  imitated  by  edible  longicorn 

beetles  and  grasshoppers Following 

PLATE  74.     Several   species   of  flies  and   the   bees   and   wasps  which   they 

imitate    .         ... Following 

PLATE  75.  A.  Aggressive  coloration  in  a  spider  (Misumena  vatia}. 
B  and  C.  "Tree-hoppers"  which  imitate  leaf-cutting  ants 
with  their  bits  of  leaves.  (In  color)  .  .  .  Following 

FIG.  38.     A  spider  which  imitates  an  ant 

PLATE  76.  Mimicry,  and  convergence  in  warning  coloration,  among  butter- 
flies. (In  color) Following 

PLATE  77.  Convergence  in  warning  coloration,  and  mimicry  among  butter- 
flies. (In  color) Following 

PLATE  78.     Caterpillars  which  assume  "terrifying  attitudes"  when  startled. 
(In  color)  '........  Following 

FIG.  39.     Spiders  which  mimic  ants     ........ 

FIG.  40.     Caterpillar  of  the  large  elephant  hawk-moth          .... 

FIG.  41.     A  moth  (Smerinthns  ocellatd)  in  "  terrifying  attitude  " 

FIG.  42.     A  moth  from  India  {Attacus  atlas}  at  the  tips  of  whose  wings 

are  markings  resembling  those  upon  the  head  of  a  cobra 
PLATE  79.     Mimicry  in  snakes        .......  Following 

PLATE  80.     A  "honey-sucker"  or  "friar-bird"  which  is  imitated  by  an  oriole 

Following 

FIG.  43.     "Cotton-tail"  rabbit,  showing  white  patch  under  tail   . 
PLATE  81.     Antelope  showing  "  danger  signal  "      .         .         .         .Following 

PLATE  82.     "Recognition    marks"  in:   A.    Killdeer   or  ring-necked   plover. 
B.  Nighthawk        .......  Following 

PLATE  83.     Confusing  coloration  in  butterflies,  moths,  and  grasshoppers.     (In 
color)     .........  Following 

PLATE  84.     Sexual   coloration  and  mimicry  in  butterflies   and    moths.     (In 
color)      .         ...         .         .          .          .         .          .  Following 

PLATE  85.     Sexual  coloration  and  protective  coloration  in  spiders.     (In  color) 

Following 

PLATE  86.     Diagrams  of  various  flowers  to  show  the  arrangement  of  their 
parts       .     .    .'        .         .         .         .         .         .         .  Following 

FIG.  44.     Fertilization  in  the  rock-rose  (Helianthemum  marifoliuni)  . 
PLATE  87.     Plants  whose  pollen  is  carried  by  wind.     A.  New  Jersey  scrub 
pine.     B.  Fescue-grass.         .         .       .-.'..         .Following 

FIG.  45.     A  bee,  showing  hairs  on  the  head,  body,  and  legs  to  which  pollen 
grains  are  clinging          .         .         .         .... 

PLATE  88.     Partridge-berry  (Mitchella)  .         .         .         .         .  Following 

PLATE  89.     The  fertilization  of  an  orchid  by  a  wasp        ..." 


PAGE 
134 

134 
134 

I36 
I36 

138 
140 

140 
140 

140 
141 

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I48 
I49 
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150 
ISO 
152 
152 

154 

IS6 


1 60 
1 60 


LIST  OF  ILLUSTRATIONS 


XIX 


PLATE     90.     Flowers  of  Aristolochia  sipho  and  Orchis  militaris      .Following 

PLATE  91.  A.  Skeletons  of  man  and  various  apes.  B.  Pelvis  of  man  and 
various  apes  .  .  .  .  •  -.  .  .  .  Following 

PLATE  92.  A.  Teeth  of  man  and  gorilla.  B.  Cerebral  hemispheres  of  man 
and  chimpanzee"  .  .  .  ...  .  Following 

PLATE  93.  Hair  tracts  on  the  arms  and  hands  of  a  man  and  a  male  chim- 
panzee   Following 

PLATE    94.     Ears  of  various  Primates    -....." 

PLATE  95.  A.  Head  of  foetus  of  orang,  showing  pointed  ear.  B.  Ear  of  a 
man,  showing  a  point  on  the  recurved  edge.  C.  Vestigial 
tail  muscles  in  man,  abnormal  ....  Following 

PLATE  96.  A.  Vestigial  muscles  of  the  human  ear.  B.  Vermiform  appen- 
dices in  orang,  man,  and  human  foetus  .  .  .  Following 

PLATE     97.     Eyes  of  various  vertebrates,  showing  the  nictitating  membrane  . 

Following 

PLATE     98.     Embryos  of  various  vertebrates  .  " 

PLATE  99.  Foot  position  and  curvature  of  spinal  column  in  gorilla,  adult 
man,  and  human  infant  ......  Following 

PLATE  100.     Foot  position  and  strength  of  grip  in  human  infants    .         " 
FIG.     46.     Early  development  of  Sacciilina  carcini      .         .         .         .         . 

PLATE  101.     Development  of  Sacculina  carcini      .         .         .         .Following 


PAGE 

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1 68 


1 68 


1 68 
1 68 


1 68 
1 68 

170 
170 

170 
170 
188 
1 88 


INTRODUCTION 

IT  is  not  my  purpose  to  argue  in  favor  of  the  theory  of 
evolution  as  opposed  to  the  theory  of  special  creation.  The 
time  is  past  when  such  discussion  would  be  profitable.  It  is 
rather  my  wish  to  set  forth  in  brief  outline  the  evolution 
theory  and  describe  some  of  the  phenomena  which  it 
explains,  and  then  to  discuss  the  relation  of  mankind  to 
evolution. 

The  biological  sciences  have  been  the  last  to  come  to  a 
position  of  dignity  as  orderly,  self-consistent  explanations 
of  phenomena.  Supernaturalism  and  anthropomorphic  inter- 
pretations once  prevailed  in  the  whole  domain  now  claimed 
by  natural  science.  Gradually  the  so-called  physical  sciences 
were  emancipated  from  the  superstitions  that  oppressed 
them.  Galileo,  Kepler,  Newton,  and  the  more  modern 
physicists  and  chemists  have  shown  that  the  phenomena  of 
nature  are  orderly  and  self-dependent,  that  the  explanation 
of  natural  phenomena  is  to  be  sought  in  other  natural  phe- 
nomena. The  stellar  systems  of  the  universe  are  held  in 
their  proper  places  by  that  mutual  influence  they  exert  upon 
one  another  which  we  call  gravitation.  Our  own  sun  moves 
along  its  appointed  daily  course  not  because  of  the  guiding 
reins  of  the  charioteer  Apollo,  but  under  the  control  of  this 
same  omnipresent  force,  gravitation.  The  mysteries  of  chem- 
istry were  not  so  much  in  the  thought  of  men  as  were  the 
more  patent  physical  phenomena,  so  we  find  less  of  supersti- 


xxii  INTR  OD  UCTION 

tion  and  unnatural  interpretation  in  this  field,  yet  the  false 
hopes  of  the  alchemist  and  his  unscientific  methods  show 
that  even  chemistry  has  had  to  grow  away  from  a  mass  of 
ignorant  belief  that  prevented  its  being  worthy  the  name  of 
science.  «• 

But  the  biological  sciences  were  still  slower  to  come  to 
their  true  position  as  dignified  science.  Here  was  the  last 
stronghold  of  the  supernaturalist.  Thrust  out  from  the  field 
of  "  physical  science  "  it  was  in  the  phenomena  of  life  that  the 
last  stand  was  made  by  those  who  claim  that  supernatural 
agency  intervenes  in  nature  in  such  a  way  as  to  modify  the 
natural  order  of  events.1  When  Darwin  came  to  dislodge 
them  from  this,  their  last  intrenchment,  there  was  a  fight, 
intense  and  bitter,  but,  like  all  attempts  to  stay  the  progress 
of  human  knowledge,  this  final  struggle  of  the  supernatural- 
ists  was  foredoomed  to  failure.  The  theory  of  evolution  has 
taken  its  place  beside  the  other  great  conceptions  of  natural 
relations,  and  largely  through  its  establishment  biology  has 
become  truly  a  science  with  a  large  group  of  phenomena  con- 
sistently arranged  and  properly  classified.  The  discussion 
which  followed  the  publication  of  Darwin's  "  Origin  of  Spe- 
cies "  lasted  for  nearly  a  generation,  but  it  is  now  practically 
closed,  so  far  as  any  attempt  to  discredit  evolution  as  a 
true  scientific  generalization  is  concerned.  Scientists  are  no 

1  The  author  believes  that  all  nature  is  controlled  by  an  intelligent  Providence, 
and  that  every  phenomenon  of  nature  is  either  natural  or  supernatural,  according  to 
one's  point  of  view.  A  book  upon  the  philosophical  bearing  of  the  theory  of  evolu- 
tion might  treat  of  the  supernatural  aspects  of  nature.  It  is  my  purpose,  however, 
to  discuss  only  the  natural  aspects.  But  it  is  important  to  insist  that  all  our  scien- 
tific knowledge  of  natural  phenomena  points  to  the  conclusion  that  these  phenomena 
are  orderly  and  self-consistent,  and  that  the  supernatural  and  natural  are  never  in 
conflict ;  in  other  words,  that  natural  phenomena  are  capable  of  being  studied  and 
classified. 


INTRODUCTION  xxiii 

longer  questioning  the  fact  of  evolution;  they  are  busied 
rather  with  the  attempt  to  further  explore  and  more  perfectly 
understand  the  operation  of  the  factors  that  are  at  work  to 
produce  that  development  of  animals  and  plants  which  we 
call  organic  evolution.  1 

But  though  the  fact  of  organic  evolution  seems  satis- 
factorily established,  we  are  still  far  from  a  satisfactory  knowl- 
edge of  the  factors  which  are  at  work  to  produce  it,  and 
especially  are  we  ignorant  of  the  manner  of  their  operation. 
For  many  generations  to  come  there  will  be  in  this  field 
abundant  opportunity  for  profitable  study.  It  is  not  my 
purpose  to  enter  into  much  discussion  of  the  more  doubtful 
questions,  but  rather  to  give,  as  briefly  as  is  consistent  with 
clearness,  an  outline  of  the  apparently  well  established  facts 
as  to  the  theory  and  some  of  its  important  corollaries. 

By  thus  avoiding  critical  discussion  as  far  as  possible,  I 
would  not  create  the  impression  that  biologists  are  entirely 
agreed  upon  all  points  of  the  theory.  There  is  endless  dis- 
cussion of  many  phases  of  the  subject.  In  three  cases 
where  there  is  general  difference  of  opinion  upon  a  funda- 
mental point  I  have  tried  to  state  the  divergent  opinions  and 
to  show  what  seems  to  me  to  be  the  safest  conclusion,  with 
the  reasons  for  my  opinion.  Two  of  these  much  mooted 
points  are  the  degree  of  efficiency  of  natural  selection,  and 
the  inheritance  of  the  effects  of  use  and  disuse.  Another 
much  discussed  factor  in  evolution  is  sexual  selection.  This 
I  have  treated  largely  by  pictures,  showing  some  of  the  phe- 
nomena about  the  explanation  of  which  there  is  so  much  dif- 
ference of  opinion.  But  however  much  difference  of  opinion 
there  may  be  among  biologists  in  regard  to  many  subsidiary 
points  of  the  theory,  there  is  substantial  agreement  upon  the 


xxiv  INTRODUCTION 

fact  of  evolution.  Biologists  do  not  doubt  that  evolution  has 
occurred  and  is  continuing. 

It  has  seemed  best  to  develop  some  one  subdivision  of  the 
subject  a  little  more  fully  than  the  rest.  The  author  has 
chosen  the  phenomena  of  color  in  animals  for  this  fuller 
treatment,  being  led  to  this  choice  chiefly  by  the  fact  that 
these  phenomena  may  readily  be  observed  by  any  reader  in 
any  locality. 

We  will  speak  first  of  the  theory,  then  of  some  of  the  phe- 
nomena which  find  their  explanation  in  the  theory ;  we  will 
consider  the  relation  of  man  to  evolution,  and  finally  will 
refer  to  a  few  of  the  corollaries  of  the  theory  which  are  of 
general  interest. 


PART    FIRST 


ORGANIC    EVOLUTION 


I.     THE    THEORY 

2.4^/^7 

NATURAL  SELECTION 
Heredity. 

Every  one  knows  that  among  both  animals  and  plants 
the  offspring  tend  to  resemble  their  parents.  The  young 
of  a  horse  is  always  a  horse  and  never  a  zebra.  Wolves  do 
not  give  birth  to  foxes.  Sunflowers  will  not  grow  from 
thistle  seed.  Each  kind  of  animal  and  plant  breeds  true, 
as  we  say.  This  was  not  always  recognized,  as  is  illustrated 
by  the  ancient  Greek  conceptions  of  the  origin  of  animals 
from  plants,  not  only  supposed  to  have  taken  place  in  the 
original  creation  of  animals,  but  also  thought  to  be  of  con- 
tinued occasional  occurrence.  Similarly,  the  belief,  preva- 
lent during  the  Middle  Ages,  that  the  goose-barnacle  (a 
kind  of  crustacean,  Fig.  i)  transforms  into  the  barnacle- 
goose  (Fig.  2}  is  an  indication  that  at  that  time  the  inde- 
pendence of  different  species  was  not  so  clearly  recognized 
as  now.  Sylvester  Giraldus,  in  his  Relations  concerning 
Ireland,  written  in  1187,  quaintly  describes  this  remark- 
able reputed  process  as  foltows: — 

"Chap,  u,  Of  Barnacles  which  grew  from  fir  timber 
and  their  nature. 

"  There  are  likewise  here  [in  Ireland]  many  birds  called 

3 


4  ORGANIC  EVOLUTION 

barnacles,  which  nature  produces  in  a  wonderful  manner, 
out  of  her  ordinary  course.  They  resemble  the  marsh 
geese,  but  are  smaller.  Being  at  first  gummy  excrescences 
from  pine-beams  floating  on  the  water,  and  then  enclosed 
in  shells  to  secure  their  free  growth,  they  hang  by  their 


FIG.  i.  —  Goose-barnacles  (Lepas  anatifcra)  attached  to  a  floating  piece  of  wood.     Natural 
size.  —  From  Brehm's  Thierleben. 


beaks,  like  seaweeds  attached  to  the  timber.  Being  in  pro- 
cess of  time  well  covered  with  feathers,  they  either  fall  into 
the  water  or  take  their  flight  into  the  free  air,  their  nour- 
ishment and  growth  being  supplied,  while  they  are  bred 
in  this  very  unaccountable  and  curious  manner,  from  the 
juices  of  the  wood  in  the  water.  I  have  often  seen  with 
my  own  eyes  more  than  a  thousand  minute  embryos  of 


NATURAL   SELECTION 


birds  of  this  species  on  the  sea-shore,  hanging  from  one 
piece  of  timber,  covered  with  shells,  and  already  formed. 
No  eggs  are  laid  by  these  birds  .  .  .;  the  hen  never  sits 
on  eggs  in  order  to  hatch  them ;  in  no  corner  of  the  world 
are  they  seen  either  to  pair,  or  build  nests.  Hence,  in 
some  parts  of  Ireland,  bishops  and  men  of  religion  make 
no  scruple  of  eating  these  birds 
on  fasting  days,  as  not  being 
flesh,  because  they  are  not  born 
of  flesh,  but  these  men  are  curi- 
ously drawn  into  error.  For,  if 
any  one  had  eaten  part  of  the 
thigh  of  our  first  parent,  which 
was  really  flesh,  although  not 
born  of  flesh,  I  should  think 
him  not  guiltless  of  having  eaten 
flesh.  Repent,  O  unhappy  Jew." 

Again,  Sir  Robert  Murray, 
in  1676,  reports  his  observations 
of  these  phenomena  to  the  Royal 
Society  of  England  :  — 

"  In  many  shells  I  opened,  I 
found  a  perfect  Sea- Fowl ;  the 
little  Bill  like  that  of  a  Goose ; 
the  Eyes  marked ;  the  Head,  Neck,  Breast,  Wings,  Tail,  and 
Feet,  formed ;  the  Feathers  everywhere  perfectly  Shaped,  and 
Blackish  colored;  and  the  Feet  like  those  of  other  Water- 
Fowl,  to  my  best  Rememberance.  The  biggest  I  found 
upon  the  Tree,  was  but  about  the  size  of  the  Figure  [an 
inch  long]  ;  nor  did  I  ever  see  any  of  the  little  Birds  alive, 
nor  meet  with  any  Body  that  did ;  only  some  credible  Per- 


FlG.  2.  —  Gerarde's  figure  of"  Barnacles  pro- 
ducing geise."  —  From  Gerarde's  Herball. 


6  ORGANIC  EVOLUTION 

sons  have  assured  me  that  they  have  seen  some  as  big  as 
their  Fist." 

This  conception  of  the  transformation  of  barnacles  into 
geese,  remarkable  as  it  was  recognized  to  be,  was  still 
accepted  among  scientific  men  for  a  long  time.  And  why 
should  not  we  gather  figs  from  thistles  ?  why  should  not 
plants  give  rise  to  animals  as  the  Greek  philosophers  be- 
lieved? That  they  do  not  do  so  is  really  a  remarkable 
fact  which  no  one  without  experience  of  nature  could  safely 
have  predicted. 

We,  however,  have  had  sufficient  experience  of  nature 
to  affirm  with  confidence  that  animals  and  plants  do  breed 
true.  The  statement  needs  no  proof  to  our  minds. 

We  can  go  farther  and  say  that  not  only  do  plants  and 
animals,  when  they  reproduce,  give  rise  to  young  which 
belong  to  the  same  species  as  their  parents;  the  young 
resemble  usually  the  particular  individuals  from  which 
they  have  sprung.  This  is  a  fact  perfectly  familiar  to 
breeders.  Among  domestic  cattle,  for  example,  the  off- 
spring resemble  their  parents  in  such  qualities  as  size,  form, 
color,  amount  and  quality  of  milk,  in  disposition,  in  fact  in 
all  features  which  we  can  observe.  The  same  is  true  of 
all  our  domestic  animals,  and  no  less  true  of  cultivated 
plants,  and  of  both  plants  and  animals  in  their  natural 
habitat. 

We  can  accept,  then,  without  further  discussion,  the 
statement  that  plants  and  animals  (all  living  things)  breed 
true ;  that  offspring  tend  to  resemble  their  parents  in  both 
specific  characters  and  individual  peculiarities.  This  rela- 
tion between  parent  and  offspring  we  have  named  heredity. 


PLATE  2.  —  Var 


randijlorum.     [After  BR1TCHER.] 


Mr.  Britcher  collected  all  these  varieties  at  one  time  in  a  single  very  restricted  area.  Observe 
that  the  plants  differ  in  size  of  blossoms,  color  of  petals  (all  white.  A;  all  green,  C,  D,  J;  or  of 
green  and  white  in  varying  proportions,  B,  E,  F,  G,  H,  I);  shape  of  petals  (sessile,  A,  B,  C,  H,  I ; 
or  stalked  with  stalks  of  varying  lengths,  D,  E,  F,  G,  J;  broad,  A  ;  or  slender,  H)  ;  form  of  flower 
bracts  (sessile,  A,  B,  C,  D,  'E,  G,  H,  I;  or  with  long,  J,  or  short,  F,  stalks;  broad,  E,  F,  G,J\ 
or  slender,  A,  B,  C,  H)  ;  position  of  stem  leaves  (arising  from  the  base  of  the  stem,  G;  or  situ- 
ated at  different  levels  upon  the  stem,  J,  F,  H,  D,  B,  C ;  often  occurring  just  below  the  flower 
bracts,  A;  in  one  case  absent  altogether,  E)  ;  form  of  stem  leaves  (sessile,  A,  B,  C,  E;  or  with 
petioles  of  varying  lengths,  D,  F,  I,  H,J,  G  ;  slender,  //,  or  broad,  A) ;  number  of  stem  leaves 
(one,  G;  or  three,  A,  B,  C,  D,  F,  I,  J ;  or  none,  E)  ;  number  of  stalks  from  a  single  bulb 
(one,  A,  B,  C,  D,  E,  F,  G,  j  ;  two,  H,  I ;  or  in  some  cases,  not  shown,  three  may  be  found). 
The  stamens  and  pistils  also  vary  in  form  and  in  size,  B,  D,  F,  G,  J.  Probably  no  finer  example 
of  variation  in  any  plant  has  been  described. 


NATURAL   SELECTION  7 

Variation. 

Yet  however  clearly  we  see  that  offspring  tend  to  re- 
semble their  parents,  it  is  no  less  evident  that  this  resem- 
blance is  not  an  exact  one.  Among  human  kind  we  find 
excellent  illustrations  of  this  principle.  However  strong 
may  be  the  family  resemblance  between  the  different  mem- 
bers of  a  family,  still  each  has  his  or  her  own  individual 
peculiarities.  No  two  are  exactly  alike.  The  children  do 
not  exactly  resemble  each  other  or  their  parents.  These 
facts  of  individual  differences  we  group  under  the  one  term, 
variation.  We  say  that  while,  under  the  influence  of  he- 
redity, the  young  tend  to  resemble  their  parents,  because  of 
variation  this  resemblance  is  more  or  less  imperfect. 

No  one  doubts  the  existence  of  variation.  All  about  us 
we  constantly  see  illustrations  of  the  principle.  Yet  few  but 
trained  biologists  realize  how  universal  and  how  extensive  is 
variation.  All  species  of  organisms  are  always  varying  in 
every  characteristic  and  in  almost  all  directions,  and  the 
extent  of  the  variation  is  very  considerable  in  most  species. 
The  individual  plants  of  any  species  vary  in  size,  in  size  of 
the  several  parts,  in  shape  of  stem  and  roots  and  leaves,  in 
number  of  leaves  and  of  blossoms,  in  color  of  petals,  in  num- 
ber of  seeds,  and  in  hardiness,  that  is,  in  ability  to  resist 
adverse  conditions  of  heat  or  cold,  of  drouth  or  flood,  and  of 
unfavorable  soil.  In  all  features,  both  structural  and  physio- 
logical, we  find  the  individuals  of  any  species  of  plant  will 
differ  from  one  another.  Absolute  uniformity  is  not  found 
in  organic  nature  (Plate  2). 

Study  a  thousand  individuals  of  any  species  with  regard 
to  any  single  character,  and  you  will  see  how  true  this  is. 
Take  the  common  trailing  arbutus  as  an  example.  You  will 


8  ORGANIC  EVOLUTION 

find  the  greatest  difference  in  the  number  of  blossoms  in  a 
single  head ;  the  number  of  clusters  of  blossoms  on  a  single 
plant  will  vary  greatly ;  the  number  of  seeds  is  very  variable,1 
so,  also,  is  the  proportion  of  these  that  will  mature  ;  the  size, 
shape,  and  weight  of  the  seeds  vary ;  within  the  seeds  is  a 
variable  amount  of  nutriment,  and  careful  chemical  analysis 
would  show  that  this  nutriment  is  not  absolutely  constant  in 
character;  the  relative  proportions  of  the  parts  within  the 
seeds  are  by  no  means  constant,  for  in  some  seeds  the  embryo 
will  be  relatively  larger  and  the  nutrient  materials  fill  a 
smaller  space,  while  in  other  seeds  these  relations  will  be 
reversed;  in  the  minute  embryo  which  each  seed  contains 
the  relative  proportions  between  the  several  parts,  the  minia- 
ture stem  and  leaf  and  bud,  are  subject  to  much  variation; 
examine  still  more  closely,  and  you  will  find  that  in  the  cells 
of  which  any  portion  of  this  minute  embryo  is  composed 
there  is  no  uniformity  in  shape,  size,  or  structure.  The 
analysis  can  be  carried  to  any  extent,  and  still  it  will  be  found 
that  every  part  of  the  organism  is  variable,  and  that  this  vari- 
ation is  not  confined  to  a  particular  direction.  The  flowers 
of  the  arbutus,  for  example,  vary,  not  in  a  single  regard. 
They  vary  in  number,  size,  shape,  number  of  petals,  length  of 
petals,  breadth  of  petals,  thickness  of  petals,  color  of  petals, 
in  the  size  of  the  nectaries  upon  the  petals,  in  the  abun- 
dance of  the  nectar  secreted,  in  its  strength  of  fragrance,  in 
its  quality  of  fragrance,  etc.  I  have  developed  this  point  to 
the  extent  perhaps  of  wearying  the  reader,  for  it  has  not 
usually  been  sufficiently  prominent  in  the  minds  of  those 
who  are  thinking  of  the  processes  of  evolution,  and  much 
confusion  and  false  thinking  can  be  avoided  if  we  remember 

1  In  many  localities  the  trailing  arbutus  rarely  matures  seed. 


13 


14  15 

16  17 

PLATE  3.  —  Varieties  of  Paludestrma  protea.     [After  STEARNS.] 


NATURAL   SELECTION  9 

that  almost  all  sorts  of  variations  are  always  present  among 
the  individuals  of  every  species.  We  have  taken  illustrations 
from  the  plants ;  of  course  the  same  phenomena  are  found 
among  animals  (Plate  3). 

Not  only  is  variation  universal,  affecting  all  organisms 
and  all  parts  of  every  organism ;  we  find  also  that  the  degree 
of  divergence  is  really  very  great.  In  some  of  our  common 
birds,  for  example,  the  length  of  wing  varies  to  the  extent  of 
one  quarter  of  the  average  for  the  species.  So  also  with  the 
length  of  tail,  the  proportion  between  length  of  wing  and 
length  of  tail,  the  size  of  beak,  the  proportions  of  the  legs, 
feet,  toes,  and  claws,  and  many  other  characters.  Mr.  J.  A. 
Allen,  in  his  memoir  On  the  Mammals  and  Winter  Birds 
of  East  Florida,  says,  "  The  facts  of  the  case  show  that  a 
variation  of  from  fifteen  to  twenty  per  cent  in  general  size, 
and  an  equal  degree  of  variation  in  the  relative  size  of 
different  parts,  may  be  ordinarily  expected  among  specimens 
of  the  same  species  and  sex,  taken  at  the  same  locality,  while 
in  some  cases  the  variation  is  even  greater  than  this." 

Animals  and  plants  do  not  all  show  an  equal  amount  of 
variation.  Among  animals  the  domestic  goose  is  a  good 
example  of  a  species  in  which  variation  is  comparatively 
slight.  Partly  as  a  result  of  this  stability,  domestication  has 
resulted  in  the  establishment  of  but  few  breeds  of  geese. 
But  even  in  those  species  of  animals  and  plants  in  which 
there  is  the  least  variation  the  differences  between  individ- 
uals are  still  readily  noticed  upon  careful  observation. 

As  an  example  of  variation  in  color  and  color  pattern 
notice  the  frontispiece,  which  shows  thirty-five  shells  of 
Neretina  virginea  variety  minor  selected  from  a  thousand, 
most  of  which  were  gathered  by  the  author  in  the  Salt  Pond 


10  ORGANIC  EVOLUTION 

near  Port  Henderson,  Jamaica.  Among  the  shells  there  col- 
lected were  sixty-eight  distinct  varieties,  as  indicated  by  the 
color  and  the  pattern  of  their  markings.  I  know  of  no  finer 
example  of  diversity  in  color  and  color  pattern  than  is  shown 
in  these  little  shells. 

Recent  studies  have  shown  that  there  are  two  probably  quite 
distinct  types  of  variation:  (ist)  "fluctuating"  or  "unstable 
variation,"  producing  usually  comparatively  slight  divergence 
from  the  parental  character,  and  (2d)  "  stable  variation  "  or 
"  mutation,"  producing  generally  greater  divergence  from  the 
parent  type.1  To  De  Vries  belongs  the  chief  credit  for  our 
better  understanding  of  variation  of  the  mutative  sort.  Of 
most  interest  for  our  purpose  is  the  fact  that  qualities  which 
appear  through  variation  of  the  unstable  or  fluctuating  sort 
may  not  be  passed  on  by  inheritance  to  the  next  generation, 
while  qualities  which  have  arisen  through  true  mutation  are 
transmitted  to  the  offspring. 

Fluctuating  variations  are  very  common,  are  universal, 
but  they  are  apparently  without  effect  upon  evolution,  for 
they  do  not  "breed  true."  One  of  the  most  striking  illustra- 
tions of  this  fact  is  seen  in  the  attempt  of  the  Agricultural 
Experiment  Station  at  Orono,  Maine,  to  develop  a  breed  of 
chickens  which  would  lay  a  very  large  number  of  eggs. 
They  bred  for  about  twenty  years  from  young  hens,  each  with 
a  record  of  two  hundred  or  more  eggs  a  year.  Yet  they 
found  that  the  offspring  of  these  record  hens  laid  few  if 
any  more  eggs  than  did  the  offspring  of  good  hens  which  had 
a  much  lower  egg-laying  record.  Variations  in  fecundity  in 
hens  are  thus  seen  to  be  of  the  unstable  type,  and  cannot  be  used 

1  English  zoologists  use  the  terms  continuous  (=  unstable)  and  discontinuous 
{  =  stable)  variations. 


NATURAL   SELECTION  II 

as  the  basis  for  establishing  a  new  breed,  for  they  are  not 
transmitted  to  the  following  generations,  i.e.  are  not  inherited. 

On  the  other  hand,  any  quality  which  arises  as  a  result  of 
mutation  is  heritable.  One  cannot  tell  in  advance  to  which 
type  any  newly  appearing  character  belongs.  Breeding  ex- 
periments are  necessary  to  determine.  Only  during  the  last  few 
years  has  the  attention  of  biologists  been  directed  to  this  dis- 
tinction between  two  types  of  variation,  yet  already  consider- 
able knowledge  of  mutation  has  been  gained.  We  know  that 
in  some  species  of  animals  and  of  plants  mutants  are  very  fre- 
quent and  of  very  diverse  sorts.  We  know  that  in  other  species 
mutation  is  more  rare.  Possibly  in  some  species  it  is  hardly 
occurring  at  all.  But  whenever  a  new  quality  arises  through 
true  mutation,  it  will  persist  in  subsequent  generations.  It  is 
in  this  way  that  new  species  characterized  by  new  qualities 
arise  —  by  mutation.  \Ve  do  not  know  in  any  case  the  cause 
of  the  mutation,  but  when  it  has  occurred  there  has  arisen 
a  new  sort  of  organism,  differing  in  one  or  more  definite  and 
heritable  qualities  from  the  individuals  in  the  parent  species, 
and  it  is  right  to  regard  the  new  organism  as  an  individual 
of  a  new  species.  To  be  sure,  the  new  species  may  be, 
probably  will  be,  very  similar  to  the  old,  generally  differing 
only  slightly  and  in  but  few  qualities,  yet  the  difference  is  a 
definite  and  constant  one  appearing  generation  after  generation. 

But  though  in  a  sense  the  mutants  belong  to  new  species, 
yet  the  fact  that  usually  they  differ  from  the  parent  species  only 
by  very  slight  characters  makes  us  hesitate  to  call  them  sepa- 
rate species.  We  often  call  them  races  or  sub-species,  re- 
serving the  term  "  species  "  for  groups  more  radically  distinct. 
It  is  as  yet  doubtful  what  terminology  will  finally  be  em- 
ployed to  designate  these  groups  of  individuals  formed  by 


12  ORGANIC  EVOLUTION 

mutation,  which  differ  but  slightly  from  the  parent  forms. 
Extreme  mutants  (often  called  sports)  may  differ  so  widely 
from  the  parent  forms  that  one  does  not  hesitate  to  class 
them  in  distinct  species.  Fluctuating  variations,  non-heritable, 
are  much  more  frequent  than  mutations,  but  mutations  are 
very  common  in  nearly  all  species  of  animals  and  plants,  as  is 
shown  by  the  ease  with  which  animals  and  plants  become 
modified  into  new  breeds  (species)  when  domesticated.  Of 
course  only  heritable  variations,  that  is,  only  mutations,  can 
serve  as  the  basis  for  establishing  new  breeds.  The  results 
obtained  from  domestication  show  us  that  mutation  must  have 
been  frequent  and  very  various,  for  the  breeds  of  our  numer- 
ous kinds  of  domestic  animals  and  plants  are  very  diverse. 

Remembering  now  these  facts  of  heredity  and  variation, 
let  us  observe  the  conditions  under  which  organisms  live,  and 
see  how  these  operate  to  cause  and  guide  their  evolution. 

The  struggle  for  existence. 

As  we  go  about  unobservant  through  the  woods  and 
fields,  glancing  carelessly  at  the  bright  flowers  and  the  birds 
busily  seeking  their  food  or  singing  in  apparent  contentment, 
or  as  we  look  over  the  ocean  and  think  of  the  fish  darting 
swiftly  through  the  clear  water,  it  all  seems  to  us  an  idyl 
of  perfect  happiness,  full  of  ease  and  play.  We  rarely  think 
of  the  constant  struggle  for  food  and  life  in  which  all  these 
trees  and  flowers,  all  the  fish  and  birds  and  other  animals, 
are  engaged.  We  fail  to  see  that  for  them  life  is  one 
continual  struggle  ;  that  the  gathering  of  food,  that  resist- 
ance to  the  unfavorable  conditions  of  climate,  cold,  drouth, 
flood,  and  storm,  that  rivalry  in  marriage  and  the  effort 
to  rear  their  young  when  born,  absorb  the  energy  of  animals 


NATURAL  SELECTION  13 

and  plants  alike;  and  that,  despite  the  strenuous  efforts 
put  forth,  the  result,  in  the  great  majority  of  cases,  is 
failure  and  death.  Yet  this  is  by  far  the  truer  picture 
of  organic  nature.  Everywhere  is  starvation  and  death, 
failure  to  reach  success  in  their  own  lives  or  in  rearing 
their  young.  To  some  this  aspect  of  nature  may  not 
seem  so  pleasant  to  contemplate,  yet  a  moment's  consid- 
eration will  show  its  truth. 

The  never  ending,  ever  stressful  struggle  for  life  is  a 
direct  and  necessary  result  of  two  facts:  first,  that  the 
amount  of  food  and  the  space  to  be  occupied  on  the  earth 
by  animals  and  plants  are  limited ;  and,  second,  that  the 
process  of  reproduction,  if  unhindered  by  any  adverse 
circumstances,  would  give  a  geometrical  ratio  of  increase 
of  plants  and  animals.  Let  us  look  a  moment  at  the  sec- 
ond of  these  two  propositions.  The  first,  that  the  earth 
is  capable  of  supporting  only  a  limited  number  of  living 
things,  is,  of  course,  understood  without  illustration,  but  the 
facts  of  geometrical  ratio  of  increase  in  animals  and  plants, 
unless  opposed  by  unfavorable  conditions,  are  worth  illus- 
trating. Our  common  American  animals  and  plants  give 
us  as  good  examples  as  we  could  wish. 

The  common  robin  raises  annually  one  to  three  broods, 
of  three  to  six  young  in  each  brood.  Say  that  the  yearly 
offspring  of  each  pair  of  birds  is  four  on  the  average, 
which  is  surely  a  low  estimate,  then  a  single  pair  of  robins 
would  have  in  the  first  generation  four  young.  The  second 
year  they  would  have  four  more  young,  and  their  young 
of  the  first  year,  mating,  would  have  eight  young,  four  for 
each  of  the  two  pairs.  If  for  ten  years  the  original  pair 
and  all  of  their  offspring  were  to  live  and  reproduce  at 


I4  ORGANIC  EVOLUTION 

the  assumed  rate,  four  young  a  year  for  each  pair  of  adults, 
then  at  the  end  of  the  tenth  year  there  would  be  over  one 
hundred  thousand  robins,  all  descendants  of  the  first  pair. 
(See  Table.) 

Adults  Young 

One  pair  of  adult  robins         ...  2 

First  year,  their  young  ....  4 

Second  year 6  12 

Third  year 18  36 

Fourth  year. 54  108 

Fifth  year     .         ._      .       .  .      -  .         .  162  324 

Sixth  year    .         .         .     .    .         .      "  ".  486  972 

Seventh  year         .         .         .         .         .  -               1,458  2,916 

Eighth  year.       -.         .        ..      ...      •;...  4,374  8,748 

Ninth  year  .         .         .         .         .         .  13,122  26,244 

Tenth  year 39,366  78,732 

End  of  tenth  year          ....  n8.,O98 

End  of  twentieth  yenr    ....  20,913,948,846 

We  see  at  once  that  the  earth  could  not  support  the 
animals  of  even  a  single  species  that  would  arise,  were  not 
the  natural  increase  of  the  species  held  in  check. 

As  a  matter  of  fact,  the  number  of  animals  or  plants  of 
any  given  species  remains  about  constant.  There  are  usu- 
ally no  great  fluctuations  from  year  to  year.  To  return, 
then,  to  our  illustration  of  the  robin,  we  can  say  that  more 
birds  (including  eggs  and  young)  die  every  year  than  live. 
If  the  whole  number  remains  constant  from  year  to  year, 
and  if  each  pair  of  robins  have  four  young  yearly,  of 
course  four  robins  die  every  year  for  each  two  that  sur- 
vive. .That  is,  the  death-rate  is  twice  as  great  as  the  total 
permanent  population. 

This  death-rate  is  greatly  surpassed  by  that  of  many 
species,  of  both  animals  and  plants,  which  have  a  much 


NATURAL   SELECTION  15 

larger  yearly  birth-rate.  Among  mammals  the  average 
birth-rate  would  perhaps  be  no  greater  than  it  is  among 
the  robins,  but  among  birds  are  many  which  have  twice 
or  three  times,  or  even  four  times,  as  many  young  each 
year  as  do  the  robins;  e.g.  the  whole  grouse  tribe,  includ- 
ing the  pheasants,  the  partridges,  and  the  quail,  also  the 
wild  jungle-fowl  from  which  our  domestic  chickens  have 
been  derived.  Snakes,  turtles,  lizards,  and  most  reptiles 
have  a  yearly  birth-rate  at  least  as  great  as  that  of  the 
more  prolific  birds.  Frogs  and  other  Amphibia  have  an 
immensely  larger  number  of  young  each  season,  often 
several  hundred  for  each  pair.  Many  of  the  fishes  lay 
half  a  million  eggs  for  each  mature  female,  so  that  here 
we  have  an  example  of  a  yearly  death-rate  two  hundred 
and  fifty  thousand  times  as  great  as  the  permanent  popu- 
lation, since  on  the  average  only  one  male  and  one  female 
out  of  this  half-million  of  young  survive  to  take  the  place 
of  their  parents  and  keep  the  number  of  individuals  in  the 
species  up  to  its  usual  mark.  A  starfish  may  lay  a  million 
eggs  each  season,  and,  as  the  number  of  adult  starfish 
remains  about  constant  from  year  to  year,  we  see  that  for 
every  starfish  living  nearly  half  a  million  die  each  year. 
The  birth-rate  among  the  Mollusca,  worms,  jellyfish, 
sponges,  and  the  Protozoa,  like  that  of  the  starfish,  is 
enormous.  Taking  animals  as  a  whole,  it  would  be  safe 
to  say  that  hundreds  of  thousands  die  every  year  for  each 
one  that  lives. 

Among  plants  the  figures  are  no  less  startling.  The 
higher  flowering  plants  reproduce  much  more  slowly  than 
most  of  the  lower  plants,  yet  among  them  the  death-rate 
is  very  large.  The  common  marguerite  daisy,  which 


1 6  ORGANIC  EVOLUTION 

grov/s  so  abundantly  in  eastern  America,  is  a  fair  ex- 
ample. It  is  a  moderate  estimate  to  say  that  one  of 
these  daisies  of  ordinary  size,  blooming  as  it  does  for 
about  two  months,  would  have  one  hundred  and  twenty- 
five  heads  of  bloom  each  year.  Each  head  of  blossoms 
would  have  about  five  hundred  seeds,  making  a  total  of 
sixty-two  thousand  five  hundred  seeds  for  each  plant  each 
year.  Of  this  number,  sixty-two  thousand  four  hundred 
and  ninety-nine,  all  but  one,  are  destined,  on  the  average, 
to  die,  even  assuming  that  the  parent  plant  dies, -which  is 
by  no  means  always  the  case.  Very  many  of  our  flower- 
ing plants  form  more  seeds  than  this  annually,  yet  their 
numbers  do  not  materially  increase  under  ordinary  con- 
ditions. 

Fern  spores  are  much  more  numerous  than  the  seeds 
of  flowering  plants,  and  the  lower  cryptogams,  the  Fungi, 
especially  the  Bacteria,  breed  with  a  rapidity  which  is  far 
beyond  our  comprehension.  Under  favorable  conditions 
a  single  bacterium  might  produce  a  million  bacteria  in  a 
day.  If  this  rate  of  increase  should  continue,  we  would 
have  at  the  end  of  a  week  a  million  million  million  million 
million  million  million"  bacteria,  all  derived  from  the  single 
individual  with  which  we  started. 

If  all  living  things  tend  to  reproduce  with  such  aston- 
ishing rapidity,  and  yet  we  find  that  their  numbers  do  not 
materially  increase,  but  remain  about  constant,  what  is  it 
that  holds  them  in  check  ?  What  kills  the  excess  ?  Many 
things,  unfavorable  conditions  of  all  sorts.  Starvation 
claims  probably  the  largest  share  of  victims ;  heat  and  cold 
kill  many ;  floods,  drouth,  and  storms  destroy  others ;  multi- 
tudes perish  to  feed  their  enemies ;  disease  takes  its  share. 


NATURAL   SELECTION  I  7 

Nature  is  fertile  in  expedients  for  killing.  Life  is  not 
easy.  Success  is  not  the  rule,  but  the  rare  exception. 
For  every  one  which  lives  and  succeeds  in  rearing  off- 
spring, thousands  and  thousands  perish.  Competition  is  so 
keen  that  no  unhealthy  or  imperfect  individual  can  endure 
it.  The  weak  fall  first,  leaving  the  field  to  their  stronger 
brethren,  who  in  turn  fight  it  out  among  themselves,  till 
finally  only  the  strongest  and  finest  survive.  In  a  struggle 
so  severe,  any  advantage,  however  slight,  of  greater  vigor, 
or  better  structure,  may  be  decisive  and  turn  the  scale. 

In  these  three  sets  of  phenomena,  heredity,  variation, 
and  the  strenuous  struggle  for  existence,  we  have  the  basis 
for  progress,  for  evolution,  by  the  survival  of  the  most  per- 
fect individuals.  Let  us  illustrate. 

Among  the  existing  individuals  of  any  species  of  animal 
or  plant  there  will  be  found,  at  any  time,  a  great  variety  of 
more  or  less  divergent  forms.  Take  as  an  example  the  com- 
mon rabbit  of  eastern  America.  Some  when  full  grown  are 
larger,  some  smaller;  some  are  swifter,  some  run  less  swiftly; 
some  are  darker  colored,  some  lighter  colored ;  some  are 
grayish,  some  more  brownish ;  some  are  more  shy  than  the 
average,  some  more  bold  than  their  fellows ;  some  are  more 
observant,  some  less  so ;  some  have  greater  endurance,  some 
diverge  to  the  other  extreme.  So  we  might  go  on.  What- 
ever character  we  choose  to  observe,  we  will  find  it  more 
strongly  developed  in  some  individuals  than  in  the  rest,  and 
conversely,  in  some  it  will  be  developed  to  less  than  the  aver- 
age degree.  The  larger  number  of  individuals  in  the  species 
will  usually  pretty  closely  agree  in  the  extent  to  which  any 
particular  character  is  developed,  but  a  considerable  number 
will  be  found  who  diverge  toward  either  extreme. 


1 8  ORGANIC  EVOLUTION 

Many  of  these  divergent  qualities  will  doubtless  have 
arisen  by  unstable  variation,  and  will  not  be  heritable.  In 
some  other  instances,  the  divergent  qualities  will  have  arisen 
by  true  mutation,  and  will  be  heritable.  Only  breeding 
experiments  could  show  to  which  class  any  particular  quality 
of  any  individual  rabbit  belongs. 

Suppose,  now,  there  be  introduced  into  the  region  where 
these  rabbits  live  some  predatory  enemy  swifter  and  more 
sly  than  those  to  which  the  rabbits  are  now  exposed. 
The  first  result  would  be  the  extermination  of  those 
rabbits  which  are  less  swift  and  less  cautious  and  observ- 
ant. Most  of  those  of  average  swiftness  and  alertness 
also  might  be  caught  and  killed.  There  would  soon  be 
left,  then,  only  the  individuals  in  which  these  valuable 
qualities  are  most  highly  developed.  They  would  persist, 
and,  escaping  their  enemies,  would  succeed  in  rearing 
young.  If  the  qualities  swiftness  and  alertness  had  arisen 
in  the  parents  through  mutation,  they  would  be  handed 
down  to  the  young,  so  that  the  young,  like  the  parents, 
would  be  swift  and  keen.  Thus,  by  the  elimination  of  the 
less  perfect  individuals  of  the  species,  there  will  have  been 
developed  a  race  of  rabbits  in  which  the  qualities  which 
aid  in  escape  from  a  swift,  keen  enemy  are  more  highly 
marked  then  in  the  former  race.  This  is  evolution  by  the 
elimination  of  the  unfit,  or  by  the  survival  of  the  fittest, 
the  process  which  is  called  natural  selection,  meaning  the 
selection  or  retention  of  the  individuals  most  perfectly 
adapted  to  the  environment  in  which  they  live.  If  on  the 
other  hand  there  had  been  present  no  true  mutations  in 
swiftness  and  keenness,  but  these  qualities,  where  especially 
marked,  had  been  mere  unstable  variations,  then  the  average 


NATURAL    SELECTION  19 

swiftness  and  average  keenness  among  the  offspring  would 
probably  be  no  greater  than  they  were  among  the  former 
rabbits  before  the  new  enemy  came,  and  so  the  whole  race 
of  rabbits  might  be  destroyed.  Many  species  have  doubt- 
less been  thus  destroyed,  when  environmental  conditions 
changed,  because  they  lacked  mutations  of  a  sort  to  adapt 
them  to  the  new  conditions.  Many  species,  on  the  other 
hand,  have  contained  individuals  showing  the  needed  muta- 
tions, and  these  have  succeeded  in  surviving  and  perpetuat- 
ing the  species  —  a  somewhat  changed  species,  to  be  sure 
—  a  more  highly  evolved  species. 

The  new  race  referred  to  in  the  illustration  chosen 
might  be  especially  characterized  not  only  by  the  two 
qualities  mentioned,  swiftness  and  keenness,  but  also  very 
likely  by  other  qualities  that  would  aid  in  escaping  the 
new  enemy,  such,  for  example,  as  more  perfect  conformity 
in  color  to  the  environment,  provided  its  conditions  of 
life  had  been  so  easy  that  perfect  color  resemblance  to  the 
environment  had  not  been  previously  a  necessity.  Several 
of  the  desired  qualities  could  perhaps  be  perfected  at  the 
same  time,  since  the  mutations  from  which  to  select  would 
not  necessarily  appear  separately  in  different  individuals,  but 
might  be  present  in  the  same  individual  at  one.,  time. 
Thus  there  would  be  found  among  our  Eastern  rabbits 
some  which  were  at  once  more  swift,  more  keen-sighted, 
more  observant,  more  shy,  more  perfectly  like  the  environ- 
ment in  color,  and  perhaps  marked  by  special  development 
of  other  desirable  qualities.  Mutation  is  more  extensive 
than  we  usually  think,  and  such  divergence  in  many  qualities 
at  once  might  perhaps  be  found.  Furthermore,  if  different 
individuals  possessing  different  desirable  qualities  survived 


20  ORGANIC  EVOLUTION 

through  the  aid  of  these  qualities,  they  might,  by  breeding 
together,  combine  all  the  desirable  qualities  in  their  offspring. 

Illustrations  of  this  principle  of  natural  selection  might 
be  indefinitely  multiplied.  The  environment  presses  upon 
the  animal  or  plant  at  all  points,  and  the  whole  organism  is 
capable  of  adaptive  response,  since  the  whole  organism 
mutates,  giving  favorable  peculiarities  for  selection.  Almost 
any  feature,  of  structure  or  of  function,  may  be  perfected 
whenever  it  becomes  desirable  to  have  it  emphasized.  The 
only  things  necessary  are  that  the  useful  character  shall 
be  present  as  a  mutation  in  some  individuals,  and  that  it  shall 
be  of  sufficient  importance  to  aid  i.ts  possessors  to  win  in  the 
struggle  for  life  in  which  they  are  constantly  engaged.  This 
struggle  is  so  severe  that  only  the  most  perfectly  endowed 
can  hope  to  win ;  so  that  an  advantage,  though  very  slight, 
may  determine  survival,  or,  as  Romanes  puts  it,  be  "of  selec- 
tion value." 

Many  individuals  with  new  qualities  which  arise  by  muta- 
tion ordinarily  live  side  by  side  with  the  other  individuals 
which  lack  these  qualities  and  possess  others.  In  a  certain 
sense  the  individuals  with  the  new  qualities  represent  a  new 
species,  but  the  difference  between  them  and  the  other  in- 
dividuals is  usually  so  slight  that  we  can  hardly  bring  our- 
selves to  accept  it  as  a  specific  difference.  It  probably  is 
better  to  call  slightly  divergent  new  types  races  or  incipient 
species,  rather  than  species.  Doubtless  nearly  every  one  of 
our  so-called  species  is  a  mixture  of  many  races  living  together 
and  interbreeding  and  so  mingling  their  racial  characters. 

There  are  two  quite  different  methods  used  by  both 
plants  and  animals  to  enable  the  several  species  to  persist 
and  not  be  destroyed  in  the  battle  of  life.  The  first  is  the 


NATURAL    SELECTION  21 

one  already  illustrated,  namely,  the  gradual  establishment,  by 
selection  of  the  most  perfect  individuals,  of  a  condition  of 
more  perfect  adaptation  of  the  individuals  of  the  species  to 
the  environment  in  which  it  lives.  The  second  is  to  so  greatly 
increase  the  number  of  the  offspring  by  great  development 
of  the  reproductive  functions,  that  from  very  numbers  they 
will  have  more  chance  of  survival.  We  can  hardly  say  that 
a  million  starfish  eggs  have  a  million  times  more  chance  of 
survival  than  would  one,  but  surely  a  starfish  that  lays  a 
million  eggs  has  much  more  likelihood  of  leaving  descendants 
than  would  one  which  laid  but  few  eggs,  other  things  of  course 
being  equal.  Most  animals  and  plants  adopt  both  methods, 
being  very  prolific  and  being  well  adapted  to  their  environment. 
Now,  evolution  is  brought  about  by  the  occurrence,  among 
the  individuals  of  a  species,  of  certain  ones  which  are  better 
fitted  for  the  life  they  are  to  live  than  are  the  others  of  the 
species ;  by  the  survival  of  these  favored  ones ;  and  by  the 
transmission  of  their  valuable  qualities  from  parent  to  off- 
spring generation  after  generation.  The  appearance  of  the 
desirable  quality  is  an  example  of  mutation :  the  survival  of 
those  individuals  which  possess  these  qualities  is  secured  by 
natural  selection :  and  the  perpetuation  of  the  useful  qualities 
is  secured  by  heredity.  It  would  seem  necessary  that,  given 
these  three  factors,  mutation,  natural  selection,  and  heredity, 
evolution  should  be  the  result.  Later  we  will  take  up  some 
of  the  most  frequently  urged  objections,  and  see  if  there  is 
any  flaw  in  this  argument. 

Before  referring  to  the  objections  to  the  theory  of  natu- 
ral selection,  let  us  notice  a  few  general  principles  in  the 
operation  of  this  factor  in  evolution. 

Note  that  natural  selection  is  a  negative  factor.     Strictly 


22  ORGANIC  EVOLUTION 

speaking,  it  produces  nothing.  It  merely  acts  upon  organ- 
isms as  they  are.  If  their  structure  or  habits  are  sufficiently 
hurtful,  they  perish.  If  they  possess  sufficiently  advantageous 
qualities,  they  win  in  the  struggle  for  existence,  while  their 
rivals  perish.  Yet  the  whole  evolution  of  organisms  along 
lines  that  adapt  them  to  their  environment  is  controlled  by 
natural  selection,  and  to  this  negative  factor  is  due  this  great- 
est and  most  salient  feature  of  organic  evolution,  its  produc- 
tion of  organisms  that  fit  their  environment. 

Observe  that  in  the  process  of  evolution  by  natural 
selection  the  welfare  of  the  individual  is  conserved  only 
so  far  as  it  contributes  to  the  welfare  of  the  race.  It  is 
necessary  that  the  more  perfect  individuals  should  survive 
long  enough  to  breed  and  hand  down  to  their  young  their 
useful  qualities,  but,  having  done  this,  their  further  life  is  a 
matter  of  indifference,  so  far  as  the  processes  of  evolution 
are  concerned.  In  case  an  animal  or  plant  has  several 
breeding  seasons  during  its  normal  life  period,  of  course 
its  preservation  until  the  completion  of  all  these  reproduc- 
tive processes  may  be  an  important  advantage  to  the  spe- 
cies, and,  if  so,  will  tend  to  be  secured;  but  in  the  case  of 
a  species  whose  members  have  but  a  single  reproductive 
period  in  a  lifetime,  as  is  the  case  with  many  insects  for 
example,  their  persistence  after  the  completion  of  the  pro- 
cesses of  reproduction  would  be  even  disadvantageous  to 
the  species,  since  they  would  consume  food  and  occupy 
space  needed  for  the  younger  individuals  which  are  to 
continue  the  species  by  reproduction.  It  is  natural  to 
find,  then,  as  we  do  among  the  insects,  the  adults  usually 
dying  after  the  breeding  season  is  over.  The  same  thing 
is  true,  of  course,  of  all  annual  plants.  Among  some  kinds 
of  animals  the  parents  care  for  the  young  after  birth,  and 


NATURAL    SELECTION 


23 


in  these  cases  it  is  easily  seen  that  the  life  of  the  parent 
will  naturally  be  continued  until  the  completion  of  the 
period  of  parental  care  over  the  offspring.  In  the  case  of 
animals  which  form  communities,  it  may  be  advantageous 
to  these  communities  to  have  their  members  continue  to  live 
even  after  their  reproductive  activity  ceases,  since  they  may 
aid  the  community  in  other  ways  than  by  reproduction. 

Let  us  see  a 
few  concrete  il- 
lustrations of  this 
principle  that  in 
the  processes  of 
natural  selection 
the  welfare  of  the 
race  and  not  of 
the  individual  is 
sought.  Very 
commonly  seen 
on  our  trees  are 
the  egg-cases  of 
the  bag-worm 
(Fig.  3),  a  moth, 
the  female  of  which  never  comes  to  complete  development, 
in  fact,  never  leaves  the  cocoon,  but  is  fertilized  by  the  male 
and  lays  her  eggs  without  ever  emerging  into  a  free  life  as 
an  active,  flying  adult.  More  than  this,  not  only  is  the 
active  life  of  the  adult  female  suppressed:  her  body  disin- 
tegrates in  the  process  of  laying  the  eggs,  so  that  ovulation 
and  the  death  of  the  female  are  simultaneous.  Here  we 
see  the  continued  existence  of  the  adult  female  after  the 
eggs  are  laid  is  of  no  value  to  the  species,  and  she  is 


FlG.  3.  —  The  "bag-worm,"  Thyroidopteryx  ephemeriformis. 

a.  Larva,  b.  Pupa.  c.  Adult  female  (wingless),  d.  Adult  male. 
e.  Longitudinal  section  of  a  cocoon  showing  the  degenerate  female 
full  of  eggs.  /  One  of  the  larvae,  showing  the  covering  of  silk  and 
twigs  in  which  the  posterior  part  of  the  body  is  enclosed,  g.  Young 
larvae,  natural  size. —  By  the  courtesy  of  the  United  States  Depart- 
ment of  Agriculture. 


24 


ORGANIC  EVOLUTION 


allowed  to  die.  The  male  in  this  same  species  is  an 
active,  flying  moth,  flight  being  necessary  in  order  that  he 
may  seek  the  female  and  fertilize  the  ova. 

Another  example  of  a  similar  sort  is  found  among  the 
bees.  Here  the  males  die  in  the  process  of  fertilizing  the 
eggs.  The  males  in  the  beehive  take  no  active  share  in 
the  work  of  the  community,  except  to  fertilize  the  eggs,  so 

that  when  this  function 
is  performed  their  con- 
tinued life  would  be  of 
no  profit  to  the  com- 
munity, in  fact  would 
be  a  positive  disadvan- 
tage, since  they  would 
use  food  and  space 
which  could  better  be 
given  to  those  indi- 
viduals who  were  of 
present  value  to  the 

FIG.  4.  —  Honey-bees  and  a  piece  of  honeycomb.  COITim  Unitv 

Still  another  ex- 
ample from  the  bees. 
The  beehive  contains  three  sorts  of  individuals  (Fig.  4) : 
the  males,  or  drones,  whose  only  function,  as  just  stated, 
is  to  fertilize  the  eggs ;  the  perfect-  female,  or  queen, 
which  lays  all  the  eggs,  usually  only  one  adult  queen  at  a 
time  being  present  in  a  normal  hive ;  and  the  workers, 
sterile  females,  who  perform  all  the  labor  of  the  hive  and 
show  the  remarkable  instincts  so  well  known  among  the 
bees.  The  workers  generally  keep  on  hand  a  number  of 
queen  larvae,  so  that  if  anything  should  destroy  the  old 


a.  Male  bee,  or  drone.  b.  Worker-bee,  a  sterile 
female,  c .  Queen  bee,  a  fertile  female.  —  From  Brehm's 
Thierleben. 


NATURAL   SELECTION  25 

queen  they  can  rear  another  queen  ;  but  they  do  not  allow 
these  larvae  to  hatch  so  long  as  the  old  queen  is  still  in 
the  hive  and  in  good  condition,  unless  swarming  is  about 
to  occur.  The  queens  have  the  bitterest  antipathy  for  one 
another,  and  should  a  new  queen  be  allowed  to  hatch 
there  would  at  once  be  a  mortal  duel  between  her  and  her 
mother,  the  old  queen.  As  this  would  not  be  conducive 
to  the  welfare  of  the  hive,  the  workers  allow  the  old  queen 
to  approach  the  cells  of  the  young  queens,  just  as  these 
are  ready  to  hatch,  and  permit  her  to  sting  them  to  death 
before  they  hatch.  Now  these  young  queens  are  partially 
encased  in  an  outer  envelope  which  is  not  easily  pierced 
by  the  sting  of  their  would-be  destroyer;  but  as  it  is  advan- 
tageous for  the  hive  that  these  unhatched  queens  should 
be  put  to  death,  we  find  that  in  forming  this  envelope 
around  themselves  they  have  left  the  posterior  part  of  their 
bodies  naked,  so  that  the  sting  of  the  adult  queen  can 
readily  penetrate  and  kill  them,  death  being  certain  when 
once  they  are  stung.  In  this  case  we  see  that  the  queen 
larvee  provide  in  their  own  structure  for  their  own  destruc- 
tion, since  this  is  for  the  advantage  of  the  communities  in 
which  they  live.  The  welfare  of  the  race,  not  of  the  indi- 
vidual, is  secured. 

As  an  example  of  communal  forms  in  which  the  con- 
tinued life  of  the  individual  members  of  the  community  is 
advantageous  to  the  community,  even  though  these  indi- 
viduals be  not  active  in  reproduction,  we  can  again  instance 
the  bees.  The  worker-bees  are  not  usually  able  to  repro- 
duce; they  are  sterile  females,  generally  incapable  of  laying 
eggs.  Yet  these  workers  are  the  most  valuable  members 
of  the  community,  carrying  on  all  the  wonderful  activities 


26  ORGANIC  EVOLUTION 

of  the  hive,  making  the  honeycomb,  gathering  and  storing 
the  honey,  rearing  the  young,  guiding  the  queen  in  the 
performance  of  her  duties,  expelling  the  males  when  the 
breeding  season  is  over,  in  fact  running  the  whole  hive. 
In  this  case  it  is  not  the  individual  worker  which  is  the  unit, 
but  the  community  in  which  it  lives,  the  hive.  It  is  the 
whole  hive,  with  all  its  mutually  helpful  members,  that  enters 
the  struggle  for  existence,  and  natural  selection  determines 
which  hives,  just  as  much  as  which  individual  bees,  shall 
survive.  There  is  selection  here  of  communities  as  well  as 
individuals  for  survival,  and  an  individual  useful  to  the 
community  for  some  other  reason  than  breeding  will  be  pre- 
served because  of  this  other  value. 

Among  human  beings  we  have  excellent  illustration  of 
the  fact  that  their  helpfulness  to  the  young  or  to  the 
community  as  a  whole  may  make  the  continued  life  of  the 
parents  of  value,  though  they  bear  no  more  children.  The 
human  child  is  very  imperfectly  developed  at  birth  ;  it  is 
dependent  on  the  parent's  care;  should  the  parent  die 
the  child  would  suffer.  The  life  of  the  parent  cannot  be 
allowed,  then,  to  cease  with  the  birth  of  the  child.  More 
than  this,  the  family  is  in  a  very  real  way  a  unit  in  the 
struggle  for  existence,  and  the  continued  life  of  its  members 
helps  the  family  to  succeed,  so  that  when  the  children  of 
the  family  shall  begin  to  rear  families  of  their  own,  they 
shall  have  an  advantageous  start  in  their  new,  semi-inde- 
pendent life.  Again  there  is  a  rivalry  between  communi- 
ties of  a  larger  sort.  Different  industrial  centres  enter  into 
•competition  with  one  another,  and  nation  contends  with 
nation  and  race  with  race.  As  the  continued  life  of  the 
individual  beyond  the  close  of  the  reproductive  period  is 


NATURAL   SELECTION  27 

of  advantage  to  these  larger  communal  units,  we  find  the 
length  of  life  is  not  determined  by  the  close  of  the  time  of 
functional  reproduction  among  men,  as  it  is  among  so  many 
of  the  lower  forms.  Still,  among  men,  as  among  other 
animals,  it  is  the  advantage  of  the  race  and  not  the  welfare 
of  the  individual  which  determines  the  length  of  life. 

This  fact,  that  among  men  the  welfare  of  the  race  is  the 
thing  secured  even  at  the  sacrifice  of  the  good  of  the  indi- 
vidual, is  clearly  seen  when  the  two  come  into  conflict.  It 
is  not  well  for  the  individual  that  he  die  in  battle,  yet, 
when  the  national  welfare  demands  it,  thousands  so  perish, 
and  there  has  even  been  developed  among  men  a  passion 
for  such  death  for  the  good  of  their  country.  When  a  man 
has  so  indulged  his  evil  impulses  that  he  has  become  a 
menace  to  the  communal  welfare,  he  is  restrained  by  a  fine, 
or  is  deprived  of  his  liberty,  or  may  even  be  killed,  and  no 
conditions  of  his  personal  welfare  are  allowed  to  interfere. 
Even  those  who  oppose  capital  punishment  do  so  chiefly 
because  they  believe  it  hurtful  to  the  community  as  a  whole. 
Altruistic  self-sacrifice  is  in  line  with  the  great  principle  in 
accordance  with  which  nature  seeks  the  welfare  of  each 
species  as  a  whole,  with  no  hesitation  because  of  any  hard- 
ship to  individuals  which  may  be  involved. 

Let  us  give  attention  to  one  other  corollary  of  the 
theory  of  natural  selection.  The  struggle  for  existence  is 
most  severe  between  those  animals  or  plants  which  seek  to 
occupy  the  same  place  in  nature.  Plants  which  live  in 
moist  valleys  may  come  into  very  severe  competition  with 
one  another,  but  they  do  not  come  into  rivalry  with  the 
plants  which  like  the  dry  hills  or  the  barren  rocks.  The 


28  ORGANIC  EVOLUTION 

individuals  of  a  single  species,  fitted  as  they  are  for  life 
under  the  same  conditions,  enter  into  the  most  constant  and 
the  most  severe  rivalry.  We  may  state  this  fact  in  another 
form  by  saying  that  the  struggle  for  existence  is  most  severe 
between  near  relatives.  Now  see  what  is  the  effect  of  this. 
We  have  a  group  of  individuals  belonging  to  the  same 
species.  Between  them  the  competition  is  more  severe  than 
is  the  rivalry  between  themselves  and  any  other  forms.  If 
now  there  arise  among  them  individuals  that  diverge,  so  as 
to  fit  them  to  occupy  a  place  slightly  different  from  that 
occupied  by  the  parent  stock,  this  will  allow  the  divergent 
forms  to  withdraw  a  little  from  the  place  where  competition 
is  most  severe,  and  so  will  give  them  a  better  chance  for  sur- 
vival. We  see  the  tendency  is  constantly  toward  divergence, 
since  divergence  lessens  the  severity  of  the  competition 
for  life.  Mutations  which  arise,  if  they  enable  their  possess- 
ors slightly  to  change  their  habit  of  life,  will  tend  to  be 
preserved,  even  though  the  place  to  which  the  divergent 
individuals  migrate  is,  in  itself,  no  better  than  the  one  they 
leave.  This,  we  see,  may  materially  affect  the  result  of  the 
process  of  evolution,  causing  forms  to  survive  which  other- 
wise would  not  be  chosen. 

Evolution,  so  far  as  it  is  dependent  upon  natural  selec- 
tion, is  more  rapid  while  the  environment  is  changing  than 
it  is  under  stable  environmental  conditions.  By  the  con- 
tinued action  of  natural  selection  animals  and  plants  become 
so  well  adjusted  to  their  environment  that  while  this  remains 
unchanged  they  undergo  comparatively  little  modification ; 
but  when  the  environment  is  changing  the  plants  and  ani- 
mals must  change  with  it,  if  they  are  to  be  well  adapted  to 


NATURAL   SELECTION  29 

their  surroundings.  Under  changing  environmental  con- 
ditions, especially  if  the  changes  be  rapid  and  considerable, 
the  more  plastic  species,  and  those  in  which  the  largest 
degree  of  variation  is  present,  will  have  a  decided  advantage 
over  their  less  readily  modified  neighbors,  and  those  species 
which  do  not  so  greatly  vary.  Many  of  the  less  plastic  and 
less  variable  species  may  be  destroyed  because  of  their  in- 
ability to  keep  pace  with  the  changes  in  their  surroundings. 
The  plasticity  of  the  organism  and  its  variation  are,  there- 
fore, important  elements,  and  the  degree  to  which  they  are 
developed  in  any  given  species  may  have  an  important 
bearing  upon  the  fate  of  that  species.  Lloyd  Morgan,  J. 
Mark  Baldwin,  and  H.  F.  Osborn  have  emphasized  the  im- 
portance of  plasticity,  showing  very  clearly  that  the  ability 
of  the  individuals  of  a  species  each  so  to  change  its  habit  or 
structure  as  to  adapt  itself  to  new  disadvantageous  conditions 
may  preserve  its  life  and  so  prevent  the  rapid  extermination 
of  the  species  when  environmental  conditions  change  for  the 
worse.  In  this  way  a  plastic  species  may  be  tided  over  a 
period  of  hurtful  environmental  changes  until  natural  selec- 
tion, preserving  such  favorable  mutations  as  are  present,  shall 
have  time  to  secure  the  fundamental  adaptation  of  the  species  to 
its  new  conditions  of  life, after  which  the  individuals  will  be  born 
in  a  condition  so  suitable  to  their  surroundings  that  they  will 
not  need  to  change  their  structure  or  natural  habits  in  order 
to  survive.  In  a  species  which  withstands  unfavorable  environ- 
mental conditions  through  the  plasticity  of  its  individual  mem- 
bers, each  individual  will  need  to  be  educated  into  harmony 
with  the  environment.  Such  individuals  of  the  species  as 
mutate  toward  greater  natural  adaptation  will  need  less  educa- 
tion. Of  course  innate  adaptation  is  more  advantageous  than 


30  ORGANIC  EVOLUTION 

adaptation  through  education,  since  it  is  immediate,  no  period 
of  disadvantage  appearing  in  the  early  life  of  the  individual. 
The  death-rate  of  the  individuals  which  become  adapted 
through  education  may  be  greater  than  that  among  the  indi- 
viduals with  more  perfect  innate  adaptation.  Thus  in  time  in- 
nate adaptation  may  be  established  for  the  species  as  a  whole. 
Mankind  are  in  all  intellectual  features  more  plastic  than 
animals  of  any  other  species.  By  education,  to  which  they 
readily  respond,  they  learn  to  so  adapt  themselves  to  un- 
favorable conditions  as  to  escape  from  much  of  the  stress 
of  the  struggle  for  existence.  They  have  learned  to  protect 
themselves  from  cold  and  inclement  weather,  from  hunger 
and  from  disease,  and  from  many  other  dangerous  elements 
in  their  environment.  Man's  great  individual  adaptability 
has  secured  his  survival,  but  at  the  same  time,  has  greatly 
hindered  his  evolution.  This  will  be  discussed  later.  It  is 
desirable  here  merely  to  observe  that  plasticity  (educability) 
in  any  species  of  organism  hinders  its  evolution  by  lessening 
the  destruction  which  lack  of  conformity  to  the  environment 
would  cause.  If  the  plasticity  is  very  marked,  as  among 
human  kind,  it  may  almost  prevent  evolution  through  natu- 
ral selection.  (Cf.  Appendix  I.) 

Artificial  selection. 

Before  leaving  the  subject  of  natural  selection  it  would 
be  well  to  refer  to  the  similar  phenomena  of  artificial  selec- 
tion. Florists  and  breeders  of  animals  use  methods  that 
very  closely  parallel  natural  selection.  We  are  familiar  with 
the  remarkable  results  which  have  been  obtained  in  the 
rearing  of  domestic  animals  and  plants.  The  many  kinds 
of  horses  in  use  (Plate  4)  are  widely  different  from  the  origi- 


E  F 

PLATE  4.  —  VARIETIES  OF  HORSES. 

A.  Thoroughbred  mare.  B.  Shire  horse  and  Shetland  pony.  C.  Arab  horse.  D.  Hackney 
mare  and  foal.  E.  Iceland  pony.  F.  New  Forest  pony  stallion.— From  Hayes'  Points  of  the 
Horse. 


PLATE  4,  a.  —  Brassica  oleracea,  L.,  the  wild  species  from  which  the  many  varieties  of 
domestic  cabbage,  kale,  cauliflower,  Brussels  sprouts,  Savoy  cabbage,  and  Swedish  turnip  have 
been  derivedi  i.  Part  of  a  flowering  and  fruiting  specimen,  two  years  old,  gathered  on  the 
rocks  near  the  sea,  Great  Orme's  Head,  Wales,  September,  1892.  2.  Another  specimen,  found 
at  the  same  place  and  at  the  same  time,  probably  three  years  old,  branched  and  bearing 
many  leafy  shoots.  Both  specimens  were  photographed  on  the  spot.  —  From  Errera  and 
Laurent,  Planches  de  Physiologie  vegetate. 


.Hcmcrrcf  .(atoosiod)  slsjl,  .sgEddBO  yovBS  ,9§fidd/3D  lo  asiJohfiv  Insioftid  —  .^,d  ,j  83TAJS 
v<(  Ijsvhsb  nsad  svcrf  doiriw  lo  \[R   ;  idfiilrfojl  bns  ,qimu)  riaibawg  ,i9woftiluBD*,aJuoiq8 
t  to  isdmam  B  zi  riairiw  ,asa»^t>  amt&t^  asiasqa  bliw 


.griof  ",l£ol  IB 
.bead  ii. 


PLATES  5,  6,  7.  —  Different  varieties  of  cabbage,  Savoy  cabbage,  kale  (borecole),  broccoli, 
Brussels  sprouts,  cauliflower,  Swedish  turnip,  and  kohlrabi ;  all  of  which  have  been  derived  by 
cultivation  from  the  wild  species  Brassica  oleracea,  which  is  a  member  of  the  Mustard  Family  or 
Cruciferce.  Irv 

Plate  5. —  i.  Cnbbage,  dark  red,  early,  pointed  head.  2.  Cabbage,  "Schweinfurt,"  spherical 
head,  large.  3.  Cabbage,  "green-glazed  American,"  no  head.  4.  Cabbage,  "sugar  loaf,"  long, 
oval  head.  5.  Cabbage,  "  Rennes  early,"  small.  6.  Savoy  cabbage,  "  Frankfurt,"  long,  oval  head. 

Plate  6. — 7.  Savoy  cabbage,  "extra  early  midsummer."  8.  Savoy  cabbage,  "Tours."  (6, 
7,  8,  differ  in  size,  shape  of  head,  and  degree  of  curling  or  crinkling  of  leaves.)  9.  Kale,  curled, 
dwarf,  sometimes  called  *' German  green."  10.  Kale,  tall,  curled,  n.  Kale,  "  marrow-stemmed." 
12.  Kale,  very-tall,  "cow  Or  tree-kale."  (In  kale  the  leaves  are  highly  developed,  but  are  not 
compacted  intQ  heads.)  , 

Plate  7. —  <3.  Brussels  sprouts,  dwarf ;  many  small  heads  along  the  stalk.  14.  Broccoli,  purple 
sprouting;  leava$.  and  blossoms  both  used.  15.  Cauliflower,  Sicilian,  "  purple  Cape  broccoli." 
16.  Cauliflower,  dwarf,  early  "Chalon."  (In  the.cauliflower  the  blossoms  are  greatly  developed, 
forming  a  compact  head.)  17.  Swedish  turnip.  In  this  type,  which  is  said  to  have  been  derived 
from  the  same  wild  species  as  the  cabbage,  the  underground  portion  of  the  stalk  has  been  enlarged. 
18.  Kohlrabi;  the  stalk  above  ground  has  been  enlarged. 


NM^^^BHBI^ 

;      ..he  wil.i  ^i-ie*  from  which  the  many  varieties  of 

V  ^  *  .  ->u«,  Savov  rahbagi-,  and  Swedish  turnip  have 

domestic  '.-•' 

been  derive!,      i    >(  ^  a    ^ 

nicks  near  liic  **a» ' :fl 
at  the  same  place  ,;-s-:  »t  .iw   -' 
many  leafy  shouts.     b-«th  »?- 
Laurent,  Planches  de  /**.- 


PLATE  5. 


PLATE  6 


PLATE  8.  —  Varieties  of  cabbage,  or  "  colewort,"  in  the  latter  part  of  the  sixteenth  century. 

a.  "  White  cabbage  cole  "  (red  cabbage  also  was  known  at  that  time),  b.  "  Open  cabbage 
cole"  (head  less  compact),  c.  "Savoy  cole."  d.  "Curled  Savoy  cole"  (leaves  and  flowers  both 
developed ;  head  of  flowers  almost  like  cauliflower),  e.  "  Cole-florie.**  f.  "  Garden  colewort " 
(kale),  g.  "  Curled  garden  cole."  A.  "  Parsley  colewort."  *'.  "Swollen  colewort."  j,  "Round 
rape  cole"  (kohlrabi).— From  Gerarde's  Herball.  Comparison  with  Plates  5.  6,  and  7  shows 
something  of  the  extent  of  modification  in  the  last  three  hundred  years. 


PLATE  9.— 
irv-cies,  Brasau 

i.  "  1 
white  M. 

5.  "'E»r' 


3d)  ni    .tiovvsloo  "  to 

nwonjl  aBW  oaljs 
rftod  aiswoB  bnB  aavBal)  "  aloo  xovB2  fashuO  "  .V>    ".aloo  X1 


lo  zaitehfiV  —  .8  3TAJ1 


'  sloo  s^BddBO  sJidW  "  .» 
e2"  .a     .(toBqntOD  asal  bBsri)  "all 
leomlB  aiawoft  to  bsarf  .'bsqolav: 
bahwO"  .^     .(sis: 
"aloo^sq 


PLATE  9. —  Varieties  of  turnips,  all  of  which  have  been  derived  by  cultivation  irom  the  wild 
species,  Brassica  rapus,  L.,  a  member  of  the  Mustard  Family. 

i.  "  Early  stone  or  stubble,"  green  top.  2.  "  Chirk  Castle  black  stone,"  dark  purple.  3.  "  Long, 
white  Meaux  or  cowhorn,"  pale  green  top.  4.  "  Early,  white,  strap-leaved  American,"  all  white. 
5.  "  Early  Vertus  or  Jersey."  Observe  how  these  varieties  differ  in  form. 


PLATE  10.  —VARIETIES  OF  DAHLIAS. 


a    Types  of  single  dahlias,    b.  "  Clifford  W.  Bruton,"  a  large,  yellow  dahlia,     c.  "  A.  D.  Livoni," 
pink,  pompon  type.     d.  A  modern  form,  red.  — From  Country  Life  in  America,  by  permissi. 
Doubleday,  Page  and  Co. 


PLATE  n.  —  A  new  "  cactus"  type  of  dahlia.    This  particular  variety  is  called  "  Kriemhilde." 
—  From  Country  Life  in  America,  by  permission  of  Doubleday,  Page  and  Co. 


NATURAL    SELECTION  31 

nal  stocks  from  which  they  were  derived.1  Our  domestic 
chickens  have  been  much  modified  from  the  jungle  fowl, 
their  ancestor.  Sheep,  cattle,  hogs,  canary  birds,  pigeons, 
and  other  kinds  of  domesticated  animals  show  similar 
modifications  of  the  original  stock.  Among  plants  we 
have*  more  numerous  instances;  for  example,  most  of  our 
garden  vegetables,  the  many  varieties  of  the  cabbage  (Plates 
5-8),  the  several  sorts  of  potatoes,  peas,  lettuce,  turnips, 
etc.  (Plate  9).  Other  instances  are  furnished  by  the  numer- 
ous kinds  of  roses,  chrysanthemums,  pansies,  tulips,  sweet- 
peas,  asters,  hollyhocks,  dahlias  (Plates  10  and  n),  and  a  host 
of  others  of  our  common  flowers  which  show  many  varieties. 
No\v,  as  just  stated,  the  methods  used  by  breeders  to 
produce  these  varieties  of  the  different  species  of  domestic 
animals  and  plants  are  closely  similar  to  the  chief  method 
adopted  by  nature  in  the  evolution  of  natural  species.  The 
breeder,  whether  of  plants  or  animals,  finding  in  his  stock  an 
individual  or  several  individuals  which  show  some  desirable 
quality,  chooses  these  individuals  to  breed  from,  and  when, 
among  their  offspring,  he  finds  some  in  which  the  useful 
quality  is  especially  pronounced,  these  again  are  chosen  for 
breeding.  The  desired  character  can  be  intensified  by  choos- 
ing, generation  after  generation,  those  individuals  in  which 
it  is  most  strongly  developed,  and  rejecting  the  others,  pro- 
vided only  that  the  desired  quality  arose  by  mutation  and  not 
by  unstable  variation.  The  breeder  rejects  the  individuals  in 
which  the  important  quality  is  weakly  developed.  So  also 
does  nature  in  the  process  of  natural  selection.  The  resem- 
blance between  the  two  processes  is  very  close,  and  the  re- 
sults are  similar.  In  the  case  of  natural  selection  we  get 

1  It  is  probable  that  domestic  horses  have  been  derived  from  several  wild  species. 


32  ORGANIC  EVOLUTION 

modification  of  the  original  stock  in  such  a  way  as  to  give 
more  perfect  conformity  to  the  environmental  conditions; 
while  in  artificial  selection  the  modification  is  such  as  to  make 
the  altered  form  more  perfectly  suit  the  uses  to  which  man 
wishes  to  put  it.  The  results  of  artificial  selection  ara  usually 
more  quickly  seen ;  for  the  selection  for  breeding  purposes  of 
individuals  with  the  desirable  qualities  is  generally  more  rigid 
than  in  nature,  where  the  more  and  the  less  adapted  forms  will 
for  a  time  breed  side  by  side,  the  more  perfect  gradually  pre- 
dominating more  and  more. 
The  extent  of  the 
modification  produced  by 
artificial  selection  is  very 
great  in  many  cases. 
Notice  the  common  do- 
mestic chickens,  in  which 
FIG.  5.— skuii  of  Polish  fowl,  showing  the  pe-  the  different  breeds  differ 

culiar  knob  that  has  been  developed  in  front  of  the 

brain  case.  —  From  Wright's  New  Book  of  Poultry,       from    One    another   to   Such 

by  the  courtesy  of  Cassell  &  Company. 

a     degree     that     if    they 

occurred  in  nature  the  several  kinds  would  be  referred  not 
only  to  different  species,  but  to  different  genera  (Plates  12-19 
and  Fig.  5).  Compare  the  slender  "game"  (Plate  12,  A; 
1 6,  B],  which  most  closely  of  all  resembles  the  ancestral 
"jungle  fowl"  (Plate  16,  A\  with  the  heavy  "  Brahma"  (Plate 
15,  C,  D}  or  "Cochin-china"  (Plate  15,  A,  B;  19,  B],  or  with 
the  long-tailed  "Japanese"  cocks  (Plate  17),  or  with  the  little 
"bantam"  (Plate  14,  D ;  19,  C).  Or  notice  the  varieties  of 
pigeons,  as  shown  in  another  illustration  (Plate  20  and  Fig.  6). 
.  These  races  differ  from  one  another  anatomically  and 
in  disposition  as  much  as  do  natural  species,  yet  in  one 
important  particular  they  fail  to  resemble  natural  species. 


A.  Malay  cock. 


B.  Colored  Dorking 


f 


C.  White  Dorking.  D.  Spanish. 

PLATE  12.— VARIETIES  OF  DOMESTIC  CHICKENS.    [Alter  TEGETMEIER.] 


B.  Houdans. 


C.  La  Fleche.  D.  White  and  game  bantams. 

PLATE  14. —  VARIETIES  OF  DOMESTIC  CHICKENS.    [Alter  TEGETMEIER.] 


A.  Partridge  Cochins. 


B.  Buff  Cochin  hen. 


C.  Dark  Brahmas.  D.  Light  Brahmas. 

PLATE  15.  — VARIETIES  OF  DOMESTIC  CHICKENS.    [After  TEGETMEIER.] 


PLATE  18.  —  A.  "Frizzled  fowls."  Many  different  kinds  of  the  ragged-feathered  chickens,  both 
bantam  and  larger  varieties,  have  been  bred.  [After  TEC.E'I  MEIER.]  B.  Head  of  Breda  cock. —  From 
Wright's  New  Book  of  Poultry,  by  the  courtesy  of  Cassell  and  Company.  C.  Head  of  salmon  foverolle, 
showing  the  peculiar  development  of  the  feathers  beneath  the  eyes  and  the  bill.  —  From  Wright's  New 
Book  of  Poultry,  by  the  courtesy  of  Cassell  and  Company. 


PLATE  ig.  —  A.  A  single  feather  from  a  "silky  fowl."  Almost  any  breed  can  be  obtained 
with  this  type  of  feathers.  [After  TEGETMEIER.]  B.  Leg  of  Cochin  cock.  All  the  feathers 
shown  are  upon  the  leg.  — From  Wright's  New  Book  of  Poultry,  by  the  courtesy  of  Cassell  and 
Company.  C.  "  Cochin  "  bantams.  [After  TEGETMEIER.] 


Pi. A  IK  20.  — VARIETIES  01 


i.  Wild  blue-rock  pigeon  (Columba  livia).  2.  Homing  pigeon.  3.  Common  mongrel  pigeon. 
4.  Archangel.  5.  Tumbler.  6.  Bald-headed  tumbler.  7.  Barb.  8.  Pouter.  9.  Russian  trumpeter. 
10.  Fairy  swallow.  n.  Black-winged  swallow.  12.  Fantail.  13.  Carrier.  14  and  15  Bluetts. 
The  bird  between  14  and  15  is  a  tailed  turbit.  —  From  a  photograph  of  an  exhibit  in  the  United 
States  National  Museum. 


NATURAL   SELECTION 


33 


They  will   often  freely   intercross   in   breeding,   while,    as   a 
usual  thing,  natural   species  will  not  do  so.     This  brings  us 


FlG.  6.  —  The  rock  pigeon  (Columda  livid)  of  northern  Africa,  from  which  the  different  vari- 
eties of  domestic  pigeons  have  been  derived  by  artificial  selection.  —  From  Brehm's  Thierleben. 

to  a  discussion    of    some    of   the    objections    urged    against 
natural  selection  as  a  widely  effective  factor  in  evolution. 

Objections  to  natural  selection  as  a  factor  in  evolution. 

To  Huxley  the  inability  of  artificial  selection  to  produce 
races  which  are  sterile  when  crossed,  seemed  the  strongest 
objection  to  the  certainty  of  effectiveness  in  natural  selec- 
tion to  produce  true  species,  which  in  nature  are  so  generally 
characterized  by  inability  to  breed  together,  or  at  least  by 
infertility  in  their  hybrid  offspring,  in  cases  in  which  hybrids 
can  be  obtained.  Doubtless  mutually  infertile  races  could 
be  produced  by  artificial  selection  if  breeders  should  care- 
fully observe  relative  degrees  of  fertility  and  select  as  pro- 


34  ORGANIC  EVOLUTION 

genitors  for  the  several  races  individuals  which  would  not 
readily  breed  with  others  than  those  of  their  own  race.  As 
a  matter  of  fact  breeders  have  not  cared  to  produce  races 
which  will  not  be  fertile  when  crossed  and  they  have  not  done 
so.  There  seems  little  doubt  that  they  could  have  done  so. 
In  nature  incipient  species  or  races  produced  by  mutation 
are  not  generally  infertile  when  crossed,  as  are  individuals  of 
two  different  major  species.  In  this  regard  natural  races 
resemble  artificially  produced  races. 

Mutual  infertility  between  certain  individuals  may  often 
in  nature  have  been  the  starting-point  in  the  divergence 
which  has  resulted  in  the  establishment  of  new  species. 
This  point  will  be  discussed  farther  on. 

Another  objection  which  has  been  urged  against  the 
efficiency  of  natural  selection  as  a  factor  in  evolution  is  the 
fact  of  the  apparent  uselessness  of  some  of  the  character- 
istics of  different  species,  both  animals  and  plants.  If  a 
character  is  useless  how  can  it  have  been  developed  by 
natural  selection,  which  operates  only  to  perpetuate  char- 
acters which  aid  their  possessors  in  the  struggle  for  exist- 
ence? First  let  us  ask,  are  useless  characters  really  found? 
Apparently  they  do  occur,  but  much  less  frequently  than 
we  would  at  first  thought  suppose.  Careful  study  often 
shows  that  structures  or  habits  apparently  useless  are  of 
real  value  to  their  possessors.  One  would  find  it  difficult 
to  give  an  instance  of  an  organ  or  characteristic  which  he 
is  sure  is  of  no  value  to  the  plant  or  animal  in  which  it  is 
found.  Yet  we  could  probably  find  such  instances.  Many 
are  familiar  with  the  beautiful  markings  on  the  shells  of 
diatoms,  a  group  of  microscopic  Alga,  or  with  the  beauti- 
fully regular  skeletons  of  many  other  microscopic  animals 


NATURAL   SELECTION  35 

and  plants.  These  shells  and  their  markings  are  often 
of  elaborate  pattern  (Plate  21);  they  are  regular  in  then- 
arrangement,  and  this  arrangement  is  constant  for  the 
species.  They  are,  then,  true  specific  characters.  Of  what 
possible  use  can  these  minute  ridges  and  furrows  upon  the 
shell,  or  the  particular  arrangement  of  skeletal  spicules,  be 
to  these  little  plants  and  animals;  or  why  are  they  more 
useful  if  regularly  arranged  according  to  a  particular  pat- 
tern ;  or  why  is  it  important  that  each  species  should  have  a 
pattern  peculiarly  its  own  ?  We  cannot  satisfactorily  answer 
these  questions.  We  know  comparatively  little  about  the 
details  of  the  life  of  these  species.  If  we  knew  more  it  is 
possible  the  explanation  of  these  skeletal  characters  might 
appear  and  we  see  that  they  are  useful.  Much  of  our 
inability  to  show  the  utility  of  the  apparently  useless  char- 
acters of  animals  and  plants  is  probably  due  to  our  ignorance 
of  the  life  habit  of  these  organisms. 

Yet  we  may,  for  the  present,  grant  that  certain  struc- 
tures and  habits  are  useless.  We  must,  however,  remem- 
ber that  natural  selection  is  not  the  only  factor  of  evolu- 
tion, and  that,  while  it  develops  directly  none  but  useful 
characters,  the  other  factors  give  rise  to  characters  that  are 
not  necessarily  useful.  This  point  will  come  out  more  clearly 
after  we  have  described  the  action  of  these  other  factors. 

But,  setting  this  point  aside,  natural  selection  may  indi- 
rectly give  rise  to  features  of  organization  or  disposition 
that  are  not  useful  to  their  possessors.  An  organism  is  a 
very  complex  thing,  with  its  parts  most  intimately  related  to 
each  other.  No  single  structure  in  the  body  is  independent 
of  the  rest.  One  part  acts  upon  another  in  ways  most 
remarkable.  The  intimacy  of  this  interrelation  of  parts  and 
the  complex  way  in  which  they  react  upon  and  influence 


36  ORGANIC  EVOLUTION 

one  another  we  have  lately  been  able  to  appreciate  more 
than  ever  before.  There  seems  to  be  some  reason  to  believe, 
though  it  is  not  yet  proven,  that  every  organ  and  cell  in  the 
body  so  acts  upon  every  other  as  to  affect  its  behavior. 

This  is  well  illustrated  by  the  effects  of  extirpation  of 
organs.  We  do  not  know  what  -effect  the  thyroid  glan.ds 
have  on  the  other  organs  of  the  human  body,  but  if  they  be 
removed  or  become  badly  diseased,  we  find  there  results  a 
profound  disturbance  of  the  functions  of  other  parts  of  the 
body,  showing  that  the  thyroid  glands  when  present  and  nor- 
mal probably  exert  some  influence  the  absence  of  which  from 
the  body  is  disastrous.  There  are  many  other  organs  whose 
functions  we  do  not  understand,  whose  extirpation  is  seriously 
injurious.  Their  influence  upon  other  organs  of  the  body 
must  be  very  important.  The  changes  which  follow  the 
destruction  of  the  organs  of  reproduction  are  of  especial 
interest  in  this  connection.  In  the  common  domestic 
chickens  the  destruction  of  the  testes  in  a  young  male  pre- 
vents the  comb  and  wattles  and  spurs  reaching  their  normal 
size,  the  habit  of  crowing  is  given  up,  the  characteristic 
combative  disposition  of  the  male  is  lost.  Likewise  the 
destruction  of  the  ovaries  in  a  young  hen  makes  the  comb 
and  wattles  enlarge,  the  habit  of  crowing  may  be  acquired, 
and  the  disposition  becomes  more  pugnacious.  Here  we 
have  a  clear  indication  that  the  presence  or  absence  of  the 
reproductive  organs  influences  organs  which  seemed  to 
casual  observation  to  be  unrelated  to  them,  namely  the  brain 
(change  of  disposition),  the  comb  and  wattles  upon  the  head, 
and  the  spurs  on  the  feet.  Probably  many  other  organs  of 
the  body  are  equally  influenced  in  ways  not  so  readily  ob- 
served. 


NATURAL   SELECTION  37 

Now  if  the  organs  of  the  body  are  so  intimately  con- 
nected with  one  another  that  what  affects  one  may  affect 
also  the  others,  we  see  at  once  that  changes  produced  by 
natural  selection  in  any  organ  of  the  body  because  of  the 
usefulness  of  such  change,  might  very  likely  bring  about 
correlated  changes  in  other  organs,  though  these  latter 
changes  be  not  in  themselves  useful.  The -secondary  modi- 
fications would  not  be  directly  due  to  natural  selection 
and  so  would  not  necessarily  have  to  be  useful.  Their 
connection  with  a  useful  modification  would  be  enough 
to  account  for  their  presence.-  This  principle  of  correlation 
is  undoubtedly  of  great  importance,  but  it  is  often  difficult 
to  understand  the  detail's  of  its  operation  in  particular  cases, 
since  the  nexus  between  the  different  organs,  postulated  by 
this  principle,  may  be  so  intimate  and  subtle  as  to  be  ex- 
ceedingly difficult  to  study. 

As  a  very  evident  example  of  correlation  think  for  a 
moment  of  the  great  weight  of  the  antlers  of  an  elk  and 
the  great  strength  required  in  the  ligamentum  nuchce,  the 
ligament  which  stretches  from  the  top  of  the  skull  along 
the  back  of  the  neck  to  the  vertebrae  between  the  shoul- 
ders. The  strength  of  this  ligament  must  have  increased 
as  the  weight  of  the  antlers  which  it  supported  increased, 
the  two  being  correlated.  In  this  instance  it  is  easy  to 
see  the  nature  of  the  connection  between  the  two  struc- 
tures, and  that  natural  selection  has  probably  produced  the 
correlation.  In  many  cases,  however,  it  is  very  difficult 
to  understand  the  relation  between  correlated  structures, 
as  in  the  case  of  the  reproductive  organs  and  the  organs 
affected  by  their  extirpation  in  the  domestic  fowl.  Wallace, 
in  his  delightful  book,  Darwinism,  says:  "In  Paraguay, 


38  ORGANIC  EVOLUTION 

horses  with  curled  hair  occur,  and  these  always  have  hoofs 
exactly  like  those  of  a  mule,  while  the  hair  of  the  mane 
and  tail  is  much  shorter  than  usual.  Now,  if  any  of 
these  characters  were  useful,  the  others  correlated  with  it 
might  be  themselves  useless,  but  would  still  be  tolerably 
constant  because  dependent  on  a  useful  organ.  So  the 
tusks  and  bristles  of  the  boar  are  correlated  and  vary  in 
development  together,  and  the  former  only  may  be  useful, 
or  both  may  be  useful  in  equal  degrees."  If,  in  case  of  the 
boar,  the  conditions  of  life  became  such  that  increase  in 
the  size  of  the  tusks  would  be  useful,  there  might  be  de- 
veloped a  race  of  boars  with  larger  tusks,  and  at  the  same 
time  the  length  and  coarseness  of  the  bristles  would 
probably  increase,  not  because  better  developed  bristles 
are  needful  in  themselves,  but  because  of  the  correlation 
between  large  tusks  and  coarse,  long  bristles,  a  correlation 
the  reason  for  which  we  are  unable  to  understand. 

In  the  case  of  the  regular  patterns  in  the  skeletons  of 
many  unicellular  animals  and  plants,  to  which  we  have  re- 
ferred, it  is  possible,  I  will  not  say  probable,  that  the  regular- 
ity of  their  arrangement  may  be  due  to  the  constitution  of 
the  protoplasm  of  the  cells  which  form  them,  to  some  regular 
arrangement  of  the  constituent  particles  of  this  protoplasm, 
especially  as  regards  its  chemical  activity,  so  that  the 
skeletons  will  be  regular,  not  because  of  any  utility  in  their 
regularity,  but  because  they  are  each  formed  by  a  bit  of 
protoplasm  so  constituted  that,  if  it  is  to  form  a  skeleton 
at  all,  it  must  form  a  regular  skeleton.  Thus  the  regular- 
ity of  the  diatom  shell  may  be  due  to  correlation  with  a 
kind  of  protoplasmic  structure  which  is  itself  useful. 

But,  though  natural   selection    is    a   factor   in  evolution, 


NATURAL    SELECTION  39 

and  even  if  it  were,  as  it  is  not,  the  only  factor,  why  should 
all  characters  of  animals  and  plants  be  useful  to  their 
possessors?  Would  not  many  chance  variations  be  pre- 
served whether  they  were  useful  or  not?  Hurtful  char- 
acters, of  course,  would  be  eliminated,  but  why  should  not 
certain  neutral  characters  persist  without  reference  to  natu- 
ral selection  ?  It  is  truly  a  remarkable  fact,  and  one  hardly 
to  have  been  anticipated,  that  so  large  a  proportion  of  the 
habits  and  structures  of  organisms  are  useful  to  their  pos- 
sessors. On  page  69  et  seq.  is  shown  one  way  in  which 
useless  characters  may  be  preserved.  [Physiological  seg- 
regation.] 

A  third  objection  urged  against  the  importance  of  the 
agency  of  natural  selection  in  evolution  is  that  certain 
organs  which  are  useful  in  their  present  condition  could 
hardly  have  been  so  when  beginning  to  form  in  the  past, 
or,  at  least  while  as  yet  very  slightly  differentiated,  could 
hardly  have  been  sufficiently  useful  to  be  of  "selection 
value,"  i.e.  to  secure  the  survival  of  the  animals  or  plants 
possessing  them.  This  is  really  a  modification  of  the 
objection  last  mentioned.  In  reply  we  may  say,  as  we  did 
in  the  last  case,  that  it  is  difficult  to  say  what  might  be 
the  usefulness  of  the  lowly  developed  organs  from  which 
the  at  present  clearjy  useful  organs  have  come  by  modifica- 
tion. If  it  is  difficult  to  determine  the  usefulness  of  an 
organ  in  a  living  animal  which  we  can  study,  how  much 
more  difficult  it  must  be  to  decide  as  to  the  usefulness  of 
an  organ  in  an  extinct  animal,  and  the  early  stages  in  the 
evolution  of  organs  at  present  useful  were  generally  passed 
through  in  animals  or  plants  of  a  kind  no  longer  found 


40  ORGANIC  EVOLUTION 

on  the  earth.  Also  the  principle  of  correlation  between 
organs  is  important  here.  Organs  not  useful  in  them- 
selves may  be  correlated  with  other  organs  of  great  value 
and  be  developed  and  perfected  along  with  these  until 
they  reach  a  degree  of  development  that  renders  them 
themselves  useful. 

There  is  another  important  principle  that  helps  us  under- 
stand the  beginnings  in  the  evolution  of  useful  structures  and 
habits.  If  some  organ  is  to  be  developed  to  meet  some  new 
need,  it  is  rarely,  if  ever,  formed  from  a  previously  undiffer- 
entiated  part  of  the  organism,  but  is  rather  formed  by  modi- 
fication of  some  organ  already  present,  the  change  in  this 
organ  fitting  it  for  a  different  use,  fitting  it  to  meet  the  new 
need.  Similarly  if  a  new  habit  needs  to  be  acquired,  it  is 
likely  to  arise  as  a  modification  of  some  previous  habit. 
The  different  stages  in  the  evolution  of  an  organ  may  each 
be  useful  for  a  different  purpose.  In  fact  it  is  probable  that 
the  organ  in  its  several  conditions .  will  serve  somewhat 
different  purposes.  One  can  hardly  mention  an  organ  in 
the  human  body,  for  example,  which  has  not  in  this  way 
been  changed  in  its  function.  The  heart  was  once  a  simple 
blood  vessel,  serving  for  the  carriage  of  blood,  not  for  its 
propulsion ;  the  lungs  were,  in  the  fishes,  the  swim-bladder, 
which  became  changed  into  an  air-breathing  organ  as  the 
terrestrial  habit  was  acquired;  the  limbs^in  the  early  aquatic 
vertebrates  probably  were  used  as  guides  and  balancers  in 
swimming  and  as  swimming  paddles,  but,  later,  as  the  terres- 
trial habit  was  acquired,  they  assumed  a  form  adapted  for  loco- 
motion on  land.  Change  of  function  and  change  of  structure 
go  hand  in  hand,  so  that  the  different  stages  in  the  evolution 
of  an  organ  do  not  all  serve  the  same  purpose.  Hair  was 


NATURAL   SELECTION  41 

derived  from  delicate  cuticular  sense  organs.  The  internal 
ears  were  probably  once  represented  by  minute  bristle-like 
organs  in  the  skin,  which  probably  were  organs  of  touch 
or  for  the  perception  of  pressure.  Remembering  this  most 
important  principle  of  change  of  function,  we  find  that  many 
apparent  difficulties  in  the  way  of  understanding  the  origin 
of  structures  in  the  body  disappear. 

But  the  chief  apparent  force  of  the  objection  that  in  their 
beginnings  organs  could  not  have  been  of  use  lies  in  the 
misconception  that  variation  is  very  slight  and  that  therefore 
any  organ  \vrould  first  appear  as  a  very  slight  modification 
and  would  progress  by  minute  stages  toward  a  condition  in 
which  it  could  be  of  use.  In  reality  mutation  is  very  con- 
siderable, so  that  a  structure  at  its  first  appearance  may 
be  sufficiently  developed  to  be  of  real  importance  to  its  pos- 
sessor. What  has  been  said  of  organs  would  apply  as  well 
to  instincts  and  other  mental  characters. 

Individuals  which  diverge  to  a  very  considerable  degree 
from  the  species  average  are  often  called  sports.  We  know 
that  some  races  or  species  have  arisen  as  sports  which  bred 
true,  handing  down  to  their  offspring  their  own  peculiar 
characters,  e.g.  Ancon  sheep.  If  this  be  true,  such  origin  of 
species  may  be  frequent,  yet  natural  selection  will  still  be 
operative  to  determine  which  of  these  new  species  shall 
survive,  only  those  persisting  which  advantageously  con- 
form to  the  environmental  conditions.  Such  sports  are 
but  extreme  mutations. 

A  fourth  objection,  which  is  related  to  the  latter  two,  is 
that  in  our  study  of  the  fossil  remains  of  extinct  animals  we 
sometimes  find  that,  as  we  pass  from  the  older  to  the  more 
recent  species,  there  is  a  progressive  series  of  modifications  of 


42  ORGANIC  EVOLU'lION 

one  or  more  organs,  showing  that  there  has  been  a  grad- 
ual, steady  change  in  a  particular  direction,  the  several  steps 
in  this  change  being  very  slight.  In  the  fossil  remains 
which  give  us  the  history  of  the  evolution  of  the  horse 
(Plates  46  and  47)  we  see  the  gradual  loss  of  the  outer  toes, 
and  a  corresponding  increase  in  size  of  the  middle  toe,  a 
gradual  increase  in  length  of  the  molar  teeth,  and  a  gradually 
increasing  complexity  of  the  ridges  on  their  grinding  sur- 
faces. It  has  been  claimed  that  the  several  steps  in  these 
modifications  are  not  of  enough  importance  to  have  given 
their  possessors  decided  advantage  in  the  struggle  for  exist- 
ence, and  that  their  progressive  development  in  these  par- 
ticular directions  must  indicate  an  inherent  tendency  to 
become  modified  in  these  directions.  If  this  progressive 
modification  in  the  ancestors  of  the  horse  be  due  to  some 
inherent  tendency  rather  than  to  natural  selection  acting 
on  a  great  number  of  all  sorts  of  variations,  selecting  only 
the  useful  ones,  then  this  casts  doubt  on  the  importance  of 
the  role  of  natural  selection  in  other  cases.  May  not  much 
of  the  evolution  of  which  we  have  evidence  be  due  to  similar, 
not  understood,  inherent  tendencies  ?  (Cf.  Orthogenesis, 
p.  49,  and  Appendix  I.) 

The  last  and  at  one  time  apparently  the  most  important 
objection,  which  we  will  mention,  to  the  idea  of  evolution 
by  means  of  natural  selection  is  this:  It  was  long  assumed 
that,  in  general,  the  offspring  of  any  pair  of  parents  tend 
to  be  somewhat  intermediate  in  character  between  the  two 
parents.1  Now  if  a  certain  favorable  variation  arise  in  but 
a  few  individuals  of  a  species,  it  seems  improbable  that  these 
divergent  individuals  will  breed  with  one  another  rather  than 

1  This  statement  needs  slight  modification,  as  will  appear  later  when  we  come  to 
the  mention  of  Mendel's  laws  in  their  relation  to  the  persistence  of  variations  (page  46). 


NATURAL   SELECTION  43 

with  the  much  more  numerous  non-divergent  members  of 
the  species.  If,  however,  a  divergent  individual  crosses  with 
a  non-divergent  individual,  the  useful  character  which  has 
appeared  in  the  divergent  individual  was  supposed  to  be 
less  marked  in  the  offspring.  In  the  following  generations 
it  would  be  still  more  diminished  by  the  same  process,  until 
finally  it  would  be  entirely  lost.  This  swamping  of  variations 
by  interbreeding  has  seemed  to  some  to  make  the  develop- 
ment of  new  characters  by  natural  selection  improbable. 

The  force  of  this  objection  is  lessened  by  our  knowledge 
of  mutation.  Qualities  which  arise  by  mutation  are  inherited 
intact  and  unmodified  by  contact  with  different  qualities  in 
the  second  individual  with  which  the  divergent  individual 
mates.  Three-quarters  of  the  offspring  of  such  a  union 
between  a  divergent  individual  A  and  a  non-aberrant  indi- 
vidual B  will  possess  the  A  qualities  unmodified,  though 
two-thirds  of  these  will  possess  also  the  corresponding  quali- 
ties of  B.  There  is  not  a  gradual  fading  out  of  the  new 
qualities,  but  these  persist  unmodified  and  uncombined  in 
one-quarter  of  the  offspring,  and  are  present  unmodified 
though  in  combination  with  B  qualities  in  twice  as  many 
more  individuals.  (See  the  discussion  of  Mendelian  'phe- 
nomena, page  46.) 

Two  individuals  of  different  species  ordinarily  will  not 
breed  together  in  a  state  of  nature,  though  occasionally  they 
will  do  so;  and  in  those  rare  cases  in  which  species  do 
cross,  the  offspring  only  very  rarely  are  fertile.  Nature,  by 
this  infertility,  has  provided  against  promiscuous  interbreed- 
ing between  species,  and  has  thus  prevented  the  species 
already  developed  from  being  lost  by  confusion  with  one 
another.  Does  she  in  some  similar  way  prevent  promis- 
cuous intercrossing  between  the  individuals  of  a  single 


44  ORGANIC  EVOLUTION 

species,  and  thus  secure  the  perpetuation  unmixed  of  favorable 
types  that  may  arise?  There  are  ways  in  which  she  might 
do  so.  In  what  ways  may  free  intercrossing  between  the 
individuals  of  the  same  species  be  prevented  ? 

In  the  first  place,  self-fertilization  is  a  most  effective  bar 
to  promiscuous  intercrossing  and  must  serve  to  perpetuate 
many  variations  that  otherwise  might  be  swamped.  This 
would  be  more  common  among  plants  than  among  the 
higher  animals,  but  it  could  occur  among  the  lower  animals, 
many  of  which  are  bisexual.  As  a  rule,  however,  at  least 
occasional  cross-fertilization  is  advantageous  and  is  often 
secured  either  by  a  reluctance  on  the  part  of  the  sperm  to 
fertilize  the  ova  of  the  same  individual,  as  is  the  case,  for 
example,  in  most  flowering  plants,  or  by  the  sperm  ripening 
either  before  or  after  the  eggs  of  the  same  individual,  so  that 
self-fertilization  cannot  occur.  Yet  self-fertilization  does  fre- 
quently occur  among  both  animals  and  plants,  and  when  it 
does  occur  it  may  allow  certain  variants  to  persist  which 
would  be  likely  to  be  swamped  by  cross-fertilization. 

Interbreeding  between  near  relatives  is  another  thing  that 
serves  to  perpetuate  and  intensify  new  characters  which  may 
appear.  This  is  the  same  thing  which  among  domestic  ani- 
mals and  plants  is  called  "  breeding  in  and  in  "  and  is  a  most 
effective  method  in  artificial  selection.  Similar  interbreeding 
between  near  relatives  among  undomesticated  forms  will  often 
be  helped  by  the  fact  that  the  individuals  of  any  species  in  a 
limited  locality  are  likely  to  be  closely  related.  An  insect, 
for  example,  lays  its  eggs  on  a  certain  food  plant.  When 
these  hatch  it  is  very  probable  that  the  males  and  females  in 
the  brood  will  mate  together  and  so  hand  down  unimpaired 
to  the  offspring  of  the  second  generation  the  characteristics 
they  received  from  their  parents.  Among  sedentary  ani- 


NATURAL    SELECTION 


45 


mals  and  plants,  and  among  those  that  are  restricted  to  a 
limited  locality,  breeding  in  and  in,  or  breeding  between  near 
relatives,  must  be  frequent  or  even  usual.  An  occasional 
cross  with  some  individual  less  closely  related  will  be  suffi- 
cient to  avoid  deleterious  effects  from  the  close  inbreeding:. 

o 

The  influence  of  locality  will  sometimes  serve  to  hinder 
swamping  of  variations  by  free  intercrossing.  The  environ- 
mental conditions  are  frequently  not  uniform  throughout  the 
whole  range  of  a  species.  Take  as  an  example  a  species  of 
plant  which  spreads  over  a  wide  area,  part  of  which  is  moist 
bottom-land,  and  part  drier  upland.  If  the  individuals  of  the 
species  vary  in  their  adaptability  to  conditions  of  moisture 
and  drouth,  as  they  almost  surely  would  do,  some  being 
better  fitted  for  life  where  moisture  is  abundant,  others  for 
life  in  drier  soil,  then  natural  selection  would,  in  each  gener- 
ation, eliminate  from  the  bottom-lands  a  large  proportion  of 
the  plants  best  fitted  for  dry  soil,  and,  conversely,  would 
destroy  on  the  dry  hills  a  large  proportion  of  the  individuals 
adapted  to  wet  soil.  Thus  in  each  locality,  in  each  genera- 
tion, the  chances  would  be  greater  of  like  breeding  with  like 
than  with  unlike.  Natural  selection,  acting  on  each  genera- 
tion separately,  would  in  this  way  raise  a  bar  to  free  inter- 
crossing of  all  mutants  in  the  species  and  would  create  a 
probability  of  like  breeding  with  like  that  would  materially 
increase  the  cumulative  effect  of  natural  selection  from  gen- 
eration to  generation. 

Variations  in  the  time  of  breeding  act  as  a  direct  bar 
to  free  intercrossing  between  the  members  of  a  species, 
those  which  mature  their  reproductive  products  at  differ- 
ent times  being,  of  course,  by  this  fact,  prevented  from 
interbreeding.  In  this  way  differences  in  breeding  season 


46  ORGANIC  EVOLUTION 

might  soon  become  definitely  established  in  two  groups  of 
the  species,  making  a  constant  distinction  which  might 
become  a  specific  character.  Now,  as  no  part  of  an  organ- 
ism varies  independently  of  the  rest,  there  would  doubt- 
less, in  establishing  the  two  groups  which  differ  in  time 
of  breeding,  also  be  established  as  constant  certain  other 
characters  associated  with  the  difference  in  breeding  time. 

Among  some  of  the  higher  animals  sexual  selection, 
that  is,  the  exercise  of  choice  in  mating,  prevents  promis- 
cuous intercrossing  and  so  may  serve  to  preserve  from 
swamping  certain  divergent  characters  which  may  be  asso- 
ciated with  such  choice.  To  this  point  we  will  refer  again. 

Anything  which  divides  a  species  into  groups  will  be 
likely  to  prevent  free  intercrossing,  and  so  tend  to  pre- 
serve characters  associated  with  the  different  groups.  We 
will  come  back  to  this  point  soon. 

The  recently  rediscovered  work  of  Mendel  has  a  bear- 
ing upon  the  question  of  the  persistence  of  variations. 
Mendel  showed  a  half-century  ago,  and  recent  workers 
have  more  fully  established,  certain  facts  of  heredity  in 
the  case  of  hybrids  between  distinct  species,  and  crosses 
between  divergent  individuals  of  the  same  species.  Castle's 
work  in  breeding  mice,  which  closely  agrees  with  Mendel's 
observations,  shows  the  point  clearly.  Castle  bred  white 
mice  and  common  gray  mice  together  and  got  the  fol- 
lowing results.  The  offspring  developed  from  the  first 
cross  were  all  apparently  normal  gray  mice.  When,  how- 
ever, a  male  and  female  from  this  first  lot  of  young  were 
bred  together  very  interesting  results  were  obtained. 
Three-fourths  of  the  young  of  this  second  lot  appeared  to 
be  normal  gray  mice,  but  one-fourth  were  found  to  be 


NATURAL    SELECTION  47 

pure  white  mice.  If  two  of  these  white  mice  were  bred 
together  they  had  white  offspring,  and  the  same  was  true 
in  breeding  again  from  their  young,  generation  after  gen- 
eration, showing  that  they  were  of  pure  strain  without 
admixture  from  the  gray  variety,  though  the  original 
parents  in  the  first  cross  were  one  gray  and  one  white. 
It  is  of  great  interest  to  note  that,  in  spite  of  the  cross- 
ing of  the  two  varieties,  there  appeared  in  the  later  gen- 
erations certain  individuals  which  were  of  pure  blood, 
showing  no  trace  of  the  admixture  which  we  would  expect 
to  find  resulting  from  the  cross.  Extensive  experiments 
in  breeding  showed  that  the  results  were  to  be  interpreted 
as  follows:  a  gray  mouse,  G,  bred  with  a  white  mouse, 
W,  gave  offspring  which  seemed  to  be  all  gray,  but  were 
really  a  mixture  of  gray  and  white,  the  gray  character 
being  dominant  and  the  white  character  obscured,  or 
"recessive,"  as  Mendel  called  it.  That  is  G  x  W  gave 
G(W\  G(W\  G(W\  etc.,  the  parenthesis  indicating  that 
the  white  character  was  recessive.  This  hidden  complex 
nature  of  the  second  generation  (the  young  from  the  first 
cross)  was  clearly  indicated  when  they  were  bred  together. 
It  was  found  that  their  offspring  were  of  three  sorts,  and 
that  these  three  kinds  were  in  definite  and  constant 
numerical  proportions.  G  (W}  x  G  (W)  gave  offspring 
i  G  +  2  G  ( W}  +  i  W,  one-fourth  being  pure  gray,  one- 
fourth  pure  white,  and  one-half  apparently  gray,  but  really, 
as  further  breeding  showed,  gray  and  white,  the  white 
character  being  recessive  and  obscured.  These  numerical 
proportions  held  true  for  an  extensive  series  of  experiments 
in  the  case  of  white  mice,  as  they  had  done  in  the  experi- 
ments of  Mendel  upon  certain  plants. 


48  ORGANIC  EVOLUTION 

We  do  not  care  here  to  discuss  in  detail  the  Mendelian 
laws,  their  cytological  explanation,  and  the  exceptions  to 
them,  though  these  subjects  are  most  interesting  and  im- 
portant. We  are  chiefly  interested,  in  the  present  connec- 
tion, in  the  fact  that  if  the  individuals  crossed  diverge  in 
qualities  that  arose  by  mutation  the  result  is  not  a  mere 
admixture  of  the  qualities  of  the  two  parents  in  the  young, 
but  that  individuals  of  pure  strain,  showing  no  admixture, 
appear  in  the  third  generation  and  in  succeeding  generations. 
Very  divergent  individuals  which  arise  by  mutation  are  com- 
monly called  "  sports."  It  is  easy  to  see  that  if  a  single  brood 
of  sports  arose  which  were  especially  well  adapted  to  their 
environment,  although  they  might  breed  with  non-divergent 
individuals  of  the  species,  yet  among  the  offspring  of  the 
third  generation  there  would  be  individuals  like  the  original 
sports.  It  might,  therefore,  be  possible  for  natural  selection 
to  change  the  character  of  the  species  from  the  old  type  to 
that  of  the  sport,  by  preserving  the  sports  and  allowing 
them  by  competition  to  destroy  the  individuals  of  the  old 
type.  Should  the  sports  prove  to  be  more  fertile  when 
crossed  with  one  another  than  when  crossed  with  individuals 
of  the  old  type,  this  would  increase  the  probability  of  the 
new  type  becoming  predominant. 

We  should  also  note  that  in  the  experiments  of  Mendel, 
and  of  others  who  have  followed  him,  the  results  stated  above 
were  not  without  exception.  For  example,  Castle  found  that 
a  certain  proportion  of  the  mice  resulting  from  the  first  cross 
of  a  gray  with  a  white  mouse  were  not  gray,  as  we  would 
have  expected  according  to  Mendel's  laws,  nor  yet  white,  but 
were  a  dappled  gray  and  white.  In  such  a  case  there  was  a 
true  mingling  of  the  characters  of  both  parents  in  the  young, 
neither  set  of  characters  predominating. 


ORTHOGENESIS 


49 


Enough  has  been  said  to  show  that  interbreeding  be- 
tween the  different  individuals  of  a  species  is  not  promis- 
cuous and  wholly  indeterminate,  and  therefore  the  favorable 
varieties  preserved  by  natural  selection  from  among  the  indi- 
viduals of  any  generation  will  not  necessarily  be  swamped 
when  these  divergent  forms  come  to  breed.  We  will  return 
to  this  subject  again.  The  phenomena  of  organic  nature 
seem  to  indicate  very  clearly  that  evolution  has  taken  place, 
and  the  evidence  points  strongly  to  natural  selection  as  a 
real  factor  and  apparently  the  chief  factor  in  this  evolution. 

ORTHOGENESIS 

Eimer  has  given  this  name  to  the  tendency  of  certain 
species  to  become  progressively  modified  along  particular 
lines.1  For  example,  the  species  of  Paludina  found  in  the 
early  tertiary  lakes  of  Slavonia  (cf.  Fig.  26,  p.  no)  became 
progressively  modified  to  have  shells  which  were  longer  and 
more  corrugated  and  with  the  apertures  more  and  more  irreg- 
ular, until  in  the  later  tertiary  the  whole  shell  had  a  decidedly 
different  appearance  from  that  of  the  early  tertiary  ancestor. 
It  seems  inconceivable  that  the  minute  steps  of  this  modi- 
fication as  they  occurred  were  all  so  useful  as  to  help  their 
possessors  to  survive  in  the  struggle  for  existence.  It  seems 
that  natural  selection  can  have  had  little  if  anything  to  do 
with  the  changes.  There  must  have  been  in  the  organisms 
themselves  an  inherent  tendency  to  modification  along  these 
lines. 

Such  tendencies  may  arise,  persist  for  a  time,  and  then 
diminish  and  cease.  They  cannot,  of  course,  produce  very 

1  St.  George  Mivart  was  the  first  to  emphasize  this  conception. 


5o  ORGANIC  EVOLUTION 

hurtful  qualities,  or  the  species  would  become  extinct,  but 
any  useful  or  indifferent  quality  may  thus  be  emphasized. 

Some  characters  in  certain  species  have  been  thus  empha- 
sized, apparently  to  a  hurtful  degree.  Examples  of  such 
probably  overemphasized  characters  are  the  great  antlers  of 
the  extinct  Irish  elk,  the  huge  tusks  of  the  mammoth  and 
mastodon,  the  great  size  of  many  extinct  mammals  and  of 
many  dinosaurs.  Possibly  the  extinction  of  these  forms  was 
hastened  by  the  overdevelopment  of  these  characteristics 
which,  though  useful  in  their  beginnings  and  probably  con- 
served by  natural  selection,  later,  when  carried  to  an  extreme, 
aided  in  the  extinction  of  their  possessors  in  the  struggle  for 
existence. 

It  is  noteworthy  that  some  kinds  of  animals,  as  the  pa- 
leontological  records  show,  became  over-ornamented,  almost 
bizarre,  just  before  their  extinction ;  examples  are  the  cephal- 
opod  molluscs,  the  trilobites,  and  the  brachiopods.  Ornamen- 
tation was  carried  far  beyond  the  bounds  of  any  conceivable 
utility. 

Osborn  has  long  emphasized  this  conception  of  inherent 
trends  in  organisms  to  evolve  along  certain  lines.  Evidence 
in  its  favor  is  constantly  increasing  as  our  knowledge  of  pale- 
ontology grows.  (Cf.  Appendix  I.) 

SEXUAL   SELECTION 

By  sexual  selection,  as  we  will  use  the  term,  is  meant  the 
exercise  of  choice  in  mating.1  Among  plants  and  lower 
animals,  if  cross-fertilization  occur  at  all,  propinquity  at  the 

1  Those  familiar  with  Darwin's  writings  will  recognize  that  I  use  the  phrase 
sexual  selection  in  a  more  limited  sense  than  does  Darwin,  following  rather  the 
usage  of  Wallace,  Lloyd  Morgan,  and  others.  For  example,  Darwin  includes  under 


SEXUAL   SELECTION  51 

time  of  reproduction  is  usually  the  thing  that  determines 
which  individuals  shall  mate  with  one  another.  Of  prefer- 
ence or  choice,  of  course,  there  is  nothing.  But  among  some 
of  the  higher  animals  there  is  evidence  that  individual  choice 
is  exercised  in  the  selection  of  mates.  Breeders  of  domestic 
animals  find  that  the  females  sometimes  prefer  certain  mates 
rather  than  others.  To  quote  Lloyd  Morgan :  "  Professor 
Low,  one  of  the  greatest  authorities  on  our  domestic  animals, 
says, '  The  female  of  the  dog,  when  not  under  restraint,  makes 
selection  of  her  mate,'  and  again,  '  The  merino  sheep  and  the 
heath  sheep  of  Scotland,  if  two .  flocks  are  mixed  together, 
each  will  breed  with  its  own  variety.'  Mr.  Darwin  has 
collected  many  facts  illustrating  this  point.  One  of  the  chief 
pigeon  fanciers  in  England  informed  him  that,  if  free  to 
choose,  each  breed  would  prefer  mating  with  its  own  kind. 
Darwin  was  informed  by  the  Rev.  W.  D.  Fox  that  his 
flocks  of  white  and  Chinese  geese  kept  distinct."  Many 
other  instances  of  preferential  mating  could  be  mentioned 
among  domestic  animals.  To  some  further  illustrations  we 
will  refer  in  connection  with  the  description  of  segregation. 
Among  wild  animals,  also,  choice  of  mates  can  be  observed. 
Phenomena  which  are  often  explained  by  sexual  selection 
are  found  in  some  kinds  of  insects,  among  spiders,  and 
among  fishes,  Amphibia,  reptiles,  birds,  and  mammals. 
Among  humankind  sexual  selection  is,  of  course,  an  impor- 
tant factor  in  evolution. 

The  birds  give  us  some  of  the  best  examples  of  sexual 

sexual  selection  the  fighting  between  the  males  for  the  possession  of  the  female, 
though  this  may  have  no  connection  with  any  exercise  of  choice  on  the  part  of  the 
female.  I  would  include  this  rather  under  natural  selection,  restricting  the  term 
sexual  selection  to  the  voluntary  choice  of  mates  by  either  the  female  or  the  male. 


52  ORGANIC  EVOLUTION 

selection.  The  males  are  usually  more  brilliant  in  plumage 
and  have  more  highly  developed  voices  than  the  females 
(Plates  22-27).  At  the  mating  season  they  parade  their 
fine  plumage  before  the  females  and  use  all  their  charms 
of  voice  to  render  themselves  attractive  to  their  desired 
mates.  They  often  go  through  the  most  remarkable  court- 
ing antics,  and  there  seems  to  be  sufficient  evidence  from 
observation  that  these  antics  and  the  brilliant  voice  and  fine 
plumage  influence  the  female  in  her  choice,  that  they  act 
as  a  sexual  excitant.  The  strutting  of  the  rooster  or  the 
turkey  cock  (Plate  27)  is  a  good  example  of  courting  habits 
among  birds  that  is  familiar  to  all  (cf.  also  Plates  23  and  24). 
Under  the  influence  of  the  courting  instinct  the  behavior  of 
many  of  our  birds  changes  its  whole  character.  The  Ameri- 
can woodcock  is  one  of  the  most  retiring  birds  we  have. 
Few  but  sportsmen  have  ever  seen  it  in  its  native  woods. 
(See  Plate  50.)  By  day  it  stays  close  'in  the  thickets,  feeding. 
It  rarely  flies  except  at  night.  It  has  no  calls  or  song.  But 
at  the  beginning  of  the  breeding  season  even  this  shy  bird 
loses  his  sedate  character  and  lightly  turns  his  fancy  to 
thoughts  of  love.  During  the  morning  and  evening  twilight 
a  male  and  female  may  come  day  after  day  to  the  same  spot 
at  the  edge  of  the  woods,  where  the  male  will  go  through  a 
series  of  performances  wholly  foreign  to  his  usual  quiet  habit. 
Chapman,  in  his  Handbook  of  Birds  of  Eastern  North 
America,  thus  describes  the  courting  of  the  woodcock: 
"  How  many  evenings  have  I  tempted  the  malaria  germs  of 
the  New  Jersey  lowlands  to  watch  the  woodcock  perform  his 
strange  sky  dance !  He  begins  on  the  ground,  with  a  formal, 
periodic  peent,  peent,  an  incongruous  preparation  for  the  wild 
rush  that  follows.  It  is  repeated  several  times  before  he 


PLATE  22.— Male  and  female  bobolink  (Dolichonyx  oryzivorus).—  ¥rom  a.  photograph  provided 
by  the  American  Museum  of  Natural  History. 


l 


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K  £ 


2  E 


! 


Ill 

111 


c  g 

•a  g  g 


i 


PLATE  25.  —  A.  Male  and  female  Nesocmtor  milo.     B.  Male  and  female  pigeon  (Phlogoenas 
jobiensis).  —  ¥*om  Gould's  Birds  of  New  Guinea. 


'LATE  27.  — Turkey  cock  "  strutting."—  From  a  mounted  specimen  in  the  United  States  National  Museum, 


SEXUAL   SELECTION  53 

springs  from  the  ground  and  on  whistling  wings  sweeps  out 
on  the  first  loop  of  a  spiral  which  may  take  him  three  hun- 
dred feet  away  from  the  ground.  Faster  and  faster  he  goes, 
louder  and  shriller  sounds  his  wing  song;  then,  after  a 
moment's  pause,  with  darting,  headlong  flight  he  pitches  in 
zigzags  to  the  earth,  uttering  as  he  falls  a  clear,  twittering 
whistle.  He  generally  returns  to  near  the  place  from  which 
he  arose,  and  the  peent  is  at  once  resumed  as  a  preparation 
to  another  round  in  the  sky." 

In  most  birds  the  males  are  colored  more  conspicuously 
than  the  females,  and  in  many  species  the  males  show 
special  development  of  certain  feathers,  or  of  spurs,  or  comb 
and  wattles,  which  are  less  marked  or  wholly  absent  in  the 
females. 

Wallace  has  called  attention  to  the  fact  that  natural 
selection  could  hardly  allow  the  females  of  the  birds,  which 
are  chiefly  occupied  in  brooding  the  eggs  and  caring  for 
the  young,  to  be  conspicuously  colored  because  of  the  dan- 
ger to  the  nest  and  young  that  would  thus  result.  It  has 
also  been  suggested  that  brilliant  coloration  in  the  male 
may  aid  him  to  serve  as  a  decoy  to  distract  attention  from 
the  female  and  the  nest.  Unfortunately  for  both  of  these 
suggestions,  some  brilliantly  colored  males  help  the  female 
in  brooding  the  'eggs  and  caring  for  the  young. 

Among  the  spiders  also  are  seen  good  examples  of 
certain  courting  colors  and  habits  (Plate  28).1  Here  the 
males  of  many  species  have  brilliantly  colored  legs  or  have 
other  portions  of  the  body  brightly  colored.  The  eyes  also 
are  like  splendid  little  jewels  of  different  shades  of  red 
and  green  and  blue.  As  the  diminutive  male  approaches 
the  often  much  larger  female,  he  advances  with  a  swaying 

1  Compare  also  Plate  85. 


54  ORGANIC  EVOLUTION 

teetering  gait,  the  bright-colored  portions  of  the  body  being 
displayed  to  the  most  advantage.  It  behooves  him  to  be 
discreet  in  his  courtship,  for,  if  he  fails  to  charm  the 
female,  he  is  likely  to  be  seized  and  devoured  by  her.  Dr. 
and  Mrs.  Peckham,  of  Milwaukee,  who  have  been  the  most 
careful  observers  of  the  hunting  spiders,  the  group  of 
spiders  in  which  courting  colors  and  courting  habits  are 
perhaps  most  developed,  are  fully  convinced  that  the 
female  is  influenced  by  the  display  of  his  charms  made 
by  the  male,  and  that  his  success  is  often  determined  by 
this  stimulus. 

Among  insects  are  found  many  instances  of  structures 
present  in  the  males  and  wanting  in  the  females  of  the 
same  species.  Stridulating  organs  for  the  production  of 
sounds  are  common  among  the  grasshoppers,  crickets,  and 
cicadas  (Plate  29,  A).  The  males  of  many  beetles  have 
enlarged  jaws  of  a  form  not  useful  for  fighting  (Plate  29,  B), 
or  hornlike  appendages  on  the  head  or  thorax,  which  are 
not  seen  in  the  females  (Plate  30 ;  Fig.  7).  In  many  species 
of  butterflies  the  males  are  decidedly  more  brilliant  than 
the  females  (Plate  84).  Bates,  speaking  of  the  butterflies 
on  the  upper  Amazon,  says:  "They  were  of  almost  all 
colors,  sizes,  and  shapes.  I  noticed  here  altogether  eighty 
species,  belonging  to  twenty-two  different 'genera.  It  is  a 
singular  fact  that,  with  a  few  exceptions,  all  the  individuals 
of  the  various  species  thus  sporting  in  sunny  places  were 
of  the  male  sex ;  their  partners,  which  are  much  more 
soberly  dressed  and  immensely  less  numerous  than  the 
males,  being  confined  to  the  shades  of  the  woods."1  (Italics 
mine.)  Again,  speaking  of  the  butterflies  of  the  whole 

1  The  Naturalist  on  the  River  Amazon. 


H  I 

PLATE  28.  — Courting  attitudes  in  hunting  spiders.     [After  G.  W.  and  E.  G.  PECKHAM.] 

A.  Marptusa  familiaris.  Left-hand  figure,  female ;  right-hand  figure,  male.  B.  Iciusmitrahts, 
male  dancing  before  female.  C,  D.  Habrocestum  howardii,  front  view  and  side  view  of  male  in 
courting  attitude.  The  first  legs  in  the  male  are  "  a  delicate,  light-green  color,  witli  a  fringe  of 
white  hairs  along  the  outer  side."  The  patella  (second  joint)  of  the  third  leg  is  enlarged,  and  its 
anterior  face  is  white,  with  a  black  spot.  The  eyes  are  brilliant.  Observe  that  the  male  assumes 
a  position  which  shows  all  of  these  features  to  best  advantage.  E.  Saitis  pulex ,  male  in  his  court- 
ing dance.  He  bends  the  legs,  first  of  one  side,  then  of  the  other,  scurrying  back  and  forth  before 
the  female,  moving  always  toward  the  side  on  which  the  legs  are  bent.  F.  Astia  vittata,  variety 
nigra,  position  of  male  approaching  female.  G,  H,  I.  Synageles  picata,  male  dancing  before 
the  female.  His  first  pair  of  legs  are  "  of  a  brilliantly  iridescent  steel-blue  color." 


B 

PLATE  29. —  A.  Male  (upper  figure)  and  female  (lower  figure)  of  seventeen-year  cicada  (Cicada 
septendecim) ,  often  inaccurately  called  "  seventeen-year  locust."  x.  Stridulating  organ  of  the  male. 
It  is  absent  in  the  female.  B.  Males  and  female  (middle  figure  above)  of  staghorn  beetle  (Lucanus 
dama) .  These  figures  illustrate  not  only  the  difference  between  the  sexes,  but  also  the  variation  in 
size  among  the  males. 


PLATE  30.  —  Male  (upper  figure)  and  female  of  the  "Hercules  beetle"  {Dynastes  hercules). 
—  From  Brehm's  Thierleben. 


PLATE  31.— Male,  female,  and  larva  of  Chaitliodes  cormitvs,  a  relative  of  the  dragon-flies. 
The  upper  figure  shows  the  male. 


SEXUAL   SELECTION 


55 


Amazon  region,  Bates  says :  "  It  is  almost  always  the  males 
only  which  are  beautiful  in  colors."  (See  also  Plates  31 
and  33,  A.) 

The  males  of  many  kinds  of   fishes  are  more  brilliantly 
colored  than    the  females,  and    in    some    species    the    males 

have  ornamental  ap- 
pendages which  are  not 
found,  or  are  less  de- 
veloped, in  the  females 
(Plate  32).  Apparently 
these  characters  are  to 
be  referred  to  sexual 
selection,  for  the  colors 
are  generally  more 
brilliant  at  the  breeding 
season,  and  the  behavior 
of  the  male  in  the  pres- 
ence of  the  female  is 
such  as  to  show  off  to 
the  best  advantage  the 
brightly  colored  parts 
of  his  body,  or  the  orna- 
mental appendages. 

In  some  of  the  Am- 
phibia the  males  are 
more  conspicuous  than 
the  females  during  the  breeding  season.  Darwin  says,  in 
his  Descent  of  Man:  "With  our  common  newts  {Triton 
punctatus  and  cristatus]  a  deep,  much  indented  crest  is 
developed  along  the  back  and  tail  of  the  male  during  the 
breeding  season,  which  disappears  during  the  winter  (Plate 


FIG.  7. —  Heads  of  male  and  female  beetles.     The 
left-hand  figures  show  the  males.     [After  DARWIN.] 

A.   Copris  isidis.      R.  Phan&us  faunus.      C.  Dipeli- 
cus  cantorl.     D.   Onthophngus  rangifer. 


56  ORGANIC  EVOLUTION 

33,  B\  Mr.  St.  George  Mivart  informs  me  that  it  is  not 
furnished  with  muscles,  and  therefore  cannot  be  used  for 
locomotion.  As  during  the  season  of  courtship  it  becomes 
edged  with  bright  colors,  there  can  hardly  be  a  doubt  that 
it  is  a  masculine  ornament.  In  many  species  the  body 
presents  strongly  contrasted,  though  lurid  tints,  and  these 
become  more  vivid  during  the  breeding  season.  The  male, 
for  instance  of  our  common  little  newt  (Triton  punctatus}, 
is  '  brownish  gray  above,  passing  into  yellow  beneath,  which 
in  the  spring  becomes  a  rich  bright  orange,  marked  every- 
where with  round  dark  spots.'  The  edge  of  the  crest  is 
then  tipped  with  bright  red  or  violet.  The  female  is  usually 
of  a  yellowish  brown  color  with  scattered  brown  dots,  and 
the  lower  surface  is  often  quite  plain." 

The  males  of  some  kinds  of  lizards  have  certain  por- 
tions of  the  body,  especially  about  the  head  and  neck, 
brightly  colored,  and  sometimes  there  are  in  these  regions 
brilliantly  iridescent  folds  of  skin  which  may  be  distended 
and  in  this  way  made  more  showy  (Plate  34).  It  is  possible 
that  these  are  used  in  attracting  the  female. 

The  mane  of  the  lion,  the  antlers  of  the  male  deer,  the 
proud  carriage  of  the  male  in  many  species  of  mammals, 
may  be  instances  of  structures  and  habits  used  in  courtship 
and  developed,  in  part,  through  sexual  selection,  though  the 
former  two  may  be  due  partly  to  natural  selection  also, 
since  they  are  of  use  in  fighting,  the  lion's  mane  as  a  pro- 
tection, the  deer's  antlers  as  weapons. 

Referring  once  more  to  the  birds,  observe  how  the  use 
of  these  special  characters  and  habits  in  the  male  is  indi- 
cated by  the  following  facts  (I  quote  from  Romanes): 
•"(«)  Male  secondary  sexual  characters  of  an  embellishing 


PLATE  32.—  ^.  Callionymus  lyra,  male  and  female.  [After  DARWIN.]  The  upper  figure 
shows  the  male.  The  lower  figure  is  more  reduced  than  the  upper.  B.  Xiphophorus  hellerii, 
male  and  female.  [After  DARWIN.]  The  upper  figure  is  the  male. 


PLATE  33.  —  A.  Male  (a)  and  female  (b}  dragon-fly  (Calopteryx  maculata).  The  wings  of  the 
male  are  a  rich  lustrous  green,  almost  black.  The  wings  of  the  female  are  very  pale  green,  almost 
colorless.  The  male  is  much  more  conspicuous.  B.  Triton  cristatus,  male,  female,  and  larva.  The 
upper  figure  is  the  male.  —  From  Brehm's  Thierleben. 


A 


14^ 


B 


PLATE  34.  —  Males  and  females  of  various  species  of  lizards.     [After  DARWIN.] 

A,  Sitana  minor,  male.     B.   C'eratophora  stoddartii,  male  and  female.     C.  Chameleo  bifurcus,  male  and 
female.     D.   Chameleo  owenii,  male  and  female. 


SEXUAL   SELECTION  57 

kind  are,  as  a  rule,  developed  only  at  maturity,  and  most 
frequently  during  only  a  part  of  the  year,  which  is  invariably 
the  breeding  season ;  (b]  they  are  always  more  or  less  seri- 
ously affected  by  emasculation ;  (c)  they  are  always,  and  only, 
displayed  in  perfection  during  the  act  of  courtship;  (d]  then, 
however,  they  are  displayed  with  the  most  elaborate  pains ; 
yet  always,  and  only,  before  the  females;  (e)  they  appear,  at 
all  events  in  many  cases,  to  have  the  effect  of  charming  the 
females  into "  accepting  the  male.  These  statements  are 
perhaps  a  little  too  emphatic,  yet  they  indicate  clearly  the 
reasons  for  believing  in  sexual  selection.  Remembering 
the  facts  of  individual  preference  in  choice  of  mates  ob- 
served among  domestic  animals  by  their  breeders,  the  real- 
ity of  sexual  selection  seems  well  established. 

Groos !  has  pointed  out  that  the  coyness  of  the  females,  in 
those  groups  of  animals  in  which  sexual  selection  occurs, 
may  be  developed  through  natural  selection.  He  says:  "As 
the  sexual  impulse  must  have  tremendous  power,  it  is  for 
the  interest  of  the  preservation  of  the  species  that  its  dis- 
charge should  be  rendered  difficult.  This  result  is  partly 
accomplished  in  the  animal  world  by  the  necessity  for  great 
and  often  long-continued  excitement  as  a  prelude  to  the 
act  of  pairing.  This  thought  at  once  throws  light  on  the 
peculiar  hereditary  arts  of  courtship,  especially  on  the  indul- 
gence in  flying,  dancing,  or  singing  by  a  whole  flock  at 
once.  But  the  hindrance  to  the  sexual  function  that  is 
most  efficacious,  though  hitherto  unappreciated,  is  the 
instinctive  coyness  of  the  female.  This  it  is  that  necessi- 
tates all  the  arts  o'f  courtship,  and  the  probability  is  that 
seldom  or  never  does  the  female  exert  any  choice.  She  is 

1  The  Play  of  Animals,  Preface. 


c^8  ORGANIC  EVOLUTION 

not  an  awarder  of  a  prize,  but  rather  a  hunted  creature.  So, 
just  as  the  beast  of  prey  has  special  instincts  for  finding  his 
prey,  the  ardent  male  must  have  special  instincts  for  subdu- 
ing feminine  reluctance,  and  just  as  in  the  beast  of  prey  the 
instinct  of  ravenous  pursuit  is  refined  into  the  various  arts 
of  the  chase,  so,  from  such  crude  efforts  at  wooing,  that 
courtship  has  finally  developed  in  which  sexual  passion  is 
psychologically  sublimated  into  love."  Groos  is  very  likely 
correct  in  his  belief  that  the  importance  of  the  act  of  pair- 
ing has  led,  through  natural  selection,  to  the  development 
of  coyness  in  the  female,  in  order  that  more  ardor  may  be 
necessitated  in  the  male  and  the  act  of  pairing  effectually 
performed.  This  belief,  however,  does  not  diminish  at  all 
the  reasons  for  recognizing  that  the  females  do  exercise 
choice.  This  choice  is  probably  not  so  much  a  conscious 
choice  between  rival  males  as  a  choice  between  accepting 
a  certain  mate  and  refusing  to  pair  at  all  with  him.  But, 
under  this  conception,  it  will  be  those  males  which  most 
successfully  stimulate  the  sexual  instincts  of  the  females 
which  will  secure  mates.  It  has  been  observed  by  Dr.  and 
Mrs.  Peckham  that  often  a  male  hunting  spider  may  fail 
to  win  the  female.  In  observing  the  courtship  of  butterflies 
I  have  found  the  male  unsuccessful  after  more  than  an  hour 
of  pursuit,  until  finally  he  has  abandoned  his  quest.  There 
seems  no  doubt  that  the  females  of  many  groups  of  animals 
do  exercise  choice,  accepting  or  rejecting  certain  mates. 

Now  observe  what  is  the  effect  of  sexual  selection  on 
evolution.  Natural  selection  secures  the  preservation  of 
characters  which  help  their  possessors  to  survive  in  the 
struggle  for  existence.1  Sexual  selection,  on  the  other  hand, 

1  This  statement  5s  not  quite  accurate,  as  we  will  see  later  (page  85),  but  it  will 
serve  for  the  present  use. 


SEXUAL   SELECTION  59 

secures  the  perpetuation  of  those  characters  in  the  male  which 
make  him  attractive  to  the  female,  irrespective  of  any  advan- 
tage or  disadvantage  in  the  struggle  for  existence.  Those 
males  which  are  attractive  will,  because  of  their  attractiveness, 
get  mates  and  have  offspring,  while  many  of  the  less  attrac- 
tive males  will  fail  to  find  mates.  In  time,  then,  through  the 
action  of  this  preference  on  the  part  of  the  females,  there 
will  be  developed  a  race  whose  males  show  the  characters 
which  are  attractive  to  the  females.  The  results  of  sexual 
selection  are  different  from  those  produced  by  natural  selec- 
tion, and  may  often  be  opposed  to  the  latter.  For  example, 
it  is  of  advantage  to  most  birds  to  be  inconspicuously  col- 
ored, so  that  they  may  more  readily  escape  their  enemies. 
Natural  selection,  therefore,  will  tend  to  produce  protec- 
tively colored  forms.  Sexual  selection,  on  the  other  hand, 
in  the  case  of  many  species,  tends  to  produce  brilliantly 
colored  males.  The  two  tendencies  are  thus  often  opposed 
to  one  another,  sometimes  one,  sometimes  the  other,  pre- 
dominating. 

Important  objections  have  been  urged  against  the  theory 
of  sexual  selection.  Many  species  of  animals  which  show 
bright  colors  or  ornaments  in  the  male  that  are  not  found  in 
the  female  are  forms  in  which  we  have  observed  no  court- 
ing habits  by  which  these  adornments  are  displayed  before 
the  female ;  and  many  of  these  are  forms  in  which  we  would 
not  expect  to  find  the  females  exercising  choice  on  the  basis 
of  the  ornamentation  pf  the  male.  Note,  for  example,  the 
beetles  (Plates  29  and  30,  and  Fig.  7)  and  certain  lowly  Crus- 
tacea (Fig.  8,  A).  If  the  peculiar  adornment  of  the  males 
in  these  species  is  due  to  something  other  than  sexual  selec- 


6o 


ORGANIC  EVOLUTION 


tion,  it  is  distinctly  possible  that  sexual  selection  may  not  be 
the  cause,  or  at  least  the  sole  cause,  of  the  adornment  of  the 
males  among  butterflies,  spiders,  fishes,  Amphibia,  lizards, 
and  birds,  in  all  of  which  courting  has  been  observed. 


FIG.  8.  —  Secondary  sexual  characters  in  copepods. 

A.  Male  of  Calocalanus  plumulosus.     B.  Female  of  Calocalanus  pavo.     C.  Male  of  the  same 
species.    [From  MORGAN,  after  GIESBRECHT.] 

Wallace  believes  that  the  greater  brilliancy  of  the  male 
or  his  possession  of  finer  voice  or  special  ornamental  ap- 
pendages is  due  to  his  greater  vigor  and  vitality,  which  is 
associated  with  his  greater  ardor. 

Groos"  has  suggested  that  the  coyness  of  the  female 
necessitates  greater  ardor  in  the  male  and  that  this  secures 


SEXUAL   SELECTION  6 1 

greater  effectiveness  in  the  act  of  pairing,  and  that  this 
difference  in  mental  character  in  the  two  sexes  has  been 
brought  about  by  natural  selection  because  of  its  usefulness, 
and  has  not  been  developed  through  the  females  choosing 
the  more  ardent  males.  (Compare  page  57.) 

Sometimes  it  is  the  female  and  not  the  male  which 
shows  the  greater  development  of  secondary  sexual  charac- 
ters (Fig.  8,  B  and  C].  In  these  forms  we  have  no  evidence 
of  the  exercise  of  choice  by  the  male  or  of  ardent  courtship 
by  the  female.  These  cases,  however,  are  rare,  and  we  do 
not  know  what  may  be  the  use  of  the  special  appendages. 

Wallace  urges  that  for  sexual  selection  to  produce  the 
results  claimed  the  less  ornamented  males  must  fail  to  find 
mates,  and,  he  says,  we  have  no  evidence  that  the  less 
adorned  males  do  fail  to  obtain  mates,  but  that,  on  the  con- 
trary, the  less  adorned  as  well  as  the  highly  ornamented 
have  offspring. 

This  statement  of  Wallace's  is  not  surely  true.  If  there 
is  a  correlation  between  vigor  and  high  development  of  the 
ornamental  sexual  characters,  as  there  is  between  vigor  and 
high  development  of  other  structures,  then,  though  the  less 
ornamented  males  may  obtain  mates,  they  are  less  vigorous 
and  will  have  less  vigorous  offspring.  If  it  be  also  true  that 
the  more  vigorous  females  are  more  sought  after  by  the 
males  than  are  their  less  vigorous  sisters,  then  they  will  have 
first  choice  of  the  males,  choosing  the  most  highly  orna- 
mented, which  are  at  the  same  time  the  more  vigorous. 
Thus  the  vigorous,  highly  ornamented  males  will  mate  with 
the  vigorous  females,  having  vigorous  offspring,  while  the 
less  ornamented  and  less  vigorous  males  will  mate  with  the 
less  vigorous  females  and  have  less  vigorous  offspring.  Nat- 


62  ORGANIC  EVOLUTION 

ural  selection  will  then  preserve  the  vigorous  offspring  of 
the  vigorous  parents,  and  the  males  among  these  will  be 
highly  ornamented  like  their  fathers.  This  is  but  conjec- 
ture. The  relations  suggested  have  not  been  established  by 
observation.  It  is  clear,  however,  that  Wallace's  statement 
is  not  self-evident. 

Morgan  keenly  suggests  an  interesting  objection.  He 
says,  "  If  in  order  to  bring  about,  or  even  maintain,  the 
results  of  sexual  selection,  such  a  tremendous  elimination  1 
of  individuals  must  take  place,  it  is  surprising  that  natural 
selection  would  not  counteract  this  by  destroying  those 
species  in  which  a  process,  so  useless  for  the  welfare  of  the 
species,  is  going  on."  ...  "  If,  in  nature,  competition  be- 
tween species  takes  place  on  the  scale  that  the  Darwinian 
theory  of  natural  selection  postulates,  such  forms,  if  they  are 
much  exposed,  would  be  needlessly  reduced  in  numbers  in 
the  process  of  acquiring  these  [ornamental]  structures  "  in 
the  male.  This  objection  of  Morgan's  is  based  upon  the 
same  assumption  as  that  of  Wallace  which  is  criticised  in 
the  preceding  paragraph. 

Prolonged  and  careful  observation,  on  a  large  scale,  of 
the  courting  and  mating  of  animals  is  needed  to  give  us  a 
sound  basis  for  judging  of  the  reality  and  degree  of  impor- 
tance of  sexual  selection.  We  do  not  even  know  from  obser- 
vation whether  the  highly  ornamented  males  are  more  suc- 
cessful in  finding  mates  than  are  their  less  adorned  fellows. 
Such  observation  is  very  difficult,  for  it  involves  keeping 
large  numbers  of  individuals  under  as  nearly  natural  condi- 
tions as  possible,  and  observing  them  continuously,  as  well 
as  keeping  complete  records  of  the  mating  and  offspring. 

1  Elimination  from  the  breeding  process. 


SEGREGATION  63 

It  is  not   surprising,  in  view  of   these    difficulties,  that   the 
statistical  records  are  very  scant. 

There  is  no  doubt  of  the  reality  and  great  importance  of 
sexual  selection  among  mankind,  and  to  the  author  its  opera- 
tion seems  probable  at  least  among  birds,  fishes,  and  spiders. 


SEGREGATION 

Natural  selection,  orthogenesis  and  sexual  selection,  and 
possibly  the  inheritance  of  parental  modifications  which  we 
will  discuss  later,  are  primary  factors  in  evolution.  Segrega- 
tion, to  which  we  have  already  made  some  reference,  is  not  a 
primary  factor  in  the  development  of  species,  but,  acting  in 
connection  with  the  primary  factors,  it  greatly  modifies  the 
results  produced  by  these.  Anything  which  divides  a 
species  into  groups  which  do  not  freely  interbreed  is  said  to 
segregate  the  members  of  the  species  into  these  subdivisions. 
In  connection  with  one  of  the  objections  urged  against  the 
effectiveness  of  natural  selection  we  spoke  of  some  of  the 
things  that  may  cause  segregation  within  a  species.  It  is 
well  to  treat  the  subject  a  little  more  fully. 

Segregation  may  be  due  to  any  of  a  number  of  causes. 
If  only  anything  operates  to  prevent  free  interbreeding 
between  any  of  the  individuals  of  a  species,  it  is  a  true 
cause  of  segregation. 

The  cause  of  segregation  may  be  geographical.  A 
species  of  wide  distribution  is  likely  to  be  divided  into 
groups,  which  do  not  habitually  interbreed,  by  the  inter- 
vention of  rivers,  or  mountain  ranges,  or  deserts,  or  oceans 
between  the  different  groups.  The  foxes  of  Europe  differ 
from  those  of  America,  and  probably  this  divergence  from 


64  ORGANIC  EVOLUTION 

their  common  ancestral  condition  was  somewhat  influenced 
by  the  fact  that  the  foxes  east  of  the  Atlantic  Ocean  were 
unable  to  breed  with  their  relatives  on  this  continent. 
The  Rocky  Mountains  have  been  a  most  effective  cause 
of  segregation  in  this  country,  and  to  their  presence  is 
due  probably  a  considerable  part  of  the  difference  between 
eastern  and  western  forms  with  common  ancestry.  The 
fauna  and  flora  of  some  of  the  islands  off  the  west  coast 
of  South  America  give  us  fine  examples  of  the  effects  of 
isolation.  We  find  the  species  distinct  from  those  on  the 
continent,  but  closely  related  to  the  latter.  It  is  hardly 
possible  that  the  island  forms  are  not  different  from  what 
they  would  have  been  if  they  had  not  been  so  separated 
from  the  continental  members  of  the  species  that  inter- 
breeding with  the  latter  was  impossible.  Even  the  species 
of  the  several  islands  within  the  Galapagos  group  are 
different,  as  is  well  illustrated  by  the  locusts  (Fig.  9).  The 
divergence  of  these  allied  species  has  not  been  due  to 
segregation  alone.  The  environmental  conditions  in  the 
different  areas  being  different,  natural  selection  must  have 
been  constantly  at  work  to  produce  differences  between 
the  individuals  residing  in  the  two  regions.  But,  though 
natural  selection  may  have  been  the  cause  of  divergence, 
we  can  readily  see  that  its  results  must  have  been  mate- 
rially affected  by  segregation.  Segregation  operates  in 
conjunction  with  the  other  factors  of  evolution. 

Another  cause  of  segregation  is  climate.  Conditions  of 
drouth  or  of  excessive  humidity,  of  heat  or  cold,  often 
raise  effective  barriers  to  the  migrations  of  both  animals 
and  plants,  and  so  segregate  widely  distributed  species  into 
groups  which  are  separate  from  one  another  so  far  as 


SEGREGATION  65 

reproduction  is  concerned.  The  faunas  and  floras  of  east- 
ern Asia  and  of  our  west  coast  give  us  possibly  the  best 
example  of  the  segregating  effect  of  climate.  At  one  time 
the  climate  of  Siberia  and  that  of  Alaska  was  semi-tropical, 
being  considerably  more  mild  than  the  present  climate  of 
Baltimore.  Of  this  we  have  abundant  evidence  in  the 
fossil  remains  of  semi-tropical  plants  and  animals  over 


FIG.  9. — Locusts  taken  on  the  Gilq«|gm  fi'-r^".  Pacific  Ocean.  AH  di «« mA  il  from  a 
common  ancestor,  bat  now  scattered  over  the  various  islands  and  differing  in  size  and  markings. 

a.  Sckistecerca  me/amor*  f  Charies  Island).  *.  -S.  imttrmedi*  fcr*«£r  (Abingdon  and  Bmdloe 
Islands),  c.  S.  intermedia  (Duncan  Island),  d.  &.  liter***  (CTnAna  Island),  e,  .S.  iirfinii 
limeata  (Albemarle  Island),  f.  S.  mtMmurm  immmemiat*  (Indefatigable  Isbad.)  The  species 
tmfermcdia  is  probably  a  hybrid  between  the  other  two  species.  — From  Jordan  and  Keiloggs 
A*im*i  Life,  by  the  courtesy  of  the  authors  and  of  D.  Appleton  &  Co. 

this  whole  area.  During  the  continuance  of  the  warm 
climate  many  species  crossed  from  Asia  to  America  and 
vice  versa  across  Behring  straits.  As  the  cold  increased, 
culminating  in  the  extreme  cold  of  the  glacial  period,  there 
was  formed  a  most  effective  barrier  to  further  migration 
from  one  continent  to  the  other,  resulting  in  the  complete 
segregation  into  two  groups  of  each  species  which  had 
representatives  in  both  regions.  We  now  find,  as  we 


66  ORGANIC  EVOLUTION 

would  expect  from  these  conditions,  that  the  Siberian 
and  western  American  faunas  and  floras,  while  having 
many  forms  which  are  closely  similar  because  of  common 
descent,  are  still  distinct,  having  very  few  species  in  com- 
mon. (Of  common  genera,  of  course,  there  are  many.) 
Natural  selection,  aided  by  segregation,  has  had  time  to 
produce  great  changes. 

Diversity  in  soil  conditions  produces  segregation  among 
plants,  and  local  differences  in  food  conditions  thus  aris- 
ing must  cause  segregation  among  animals,  different 
groups  of  a  single  species  being  found  in  the  separate 
localities  where  the  suitable  conditions  of  soil  or  food 
e&ist. 

One  of  the  finest  examples  of  extreme  segregation  within 
a  limited  area  is  furnished  by  the  land  shells  of  Oahu,  one  of 
the  Hawaiian  Islands.  Along  the  northeastern  shore  of  the 
island  is  a  high  mountain  range  whose  sides  have  by  erosion 
been  cut  into  deep  valleys  (Fig.  10)  with  high  and  steep 
ridges  between.  The  soil  in  the  lower  ground  of  each  valley 
is  rich  and  bears  a  profusion  of  tropical  trees,  shrubs,  ferns, 
and  other  plants.  The  tops  of  the  main  ridge,  however,  and 
also  the  tops  of  the  lateral  ridges,  are  barren,  being  denuded 
of  their  soil  by  the  heavy  rains.  Several  genera  of  land 
snails,  which  feed  upon  the  foliage  of  the  trees,  shrubs,  and 
herbs,  are  very  abundant  along  this  whole  series  of  valleys, 
and  it  is  interesting  to  observe  that  each  of  the  several 
species  (or  varieties  ?)  of  snails  is  confined  to  a  single  valley 
or  to  two  or  three  adjacent  valleys.  Their  proper  food  and 
the  necessary  shade  being  absent  on  the  tops  of  the  ridges, 
the  snails  do  not  cross  from  one  valley  to  the  next.  Such 
spreading  as  has  occurred  has  probably  been  due  to  the 


SEGREGATION-  67 

snails  being  transported  by  birds  or  by  some  different  means 
other  than  their  own  powers  of  locomotion.  Gulick,  who 
first  called  attention  to  the  importance  of  segregation  as  a 


FIG.  10.  —  Map  of  Oahu,  one  of  the  Hawaiian  Islands. 

factor  in  evolution,  was  led  to  his  conclusions  by  his  study 
of  the  remarkably  restricted  range  of  each  of  the  many 
species  of  land  snails  in  these  Oahu  mountain  gorges. 

The  difficulties  in  the  way  of  migration  over  great  dis- 
tances must  tend  toward  segregation  among  both  plants  and 
animals.  The  individuals  at  the  extremes  of  the  area  occu- 
pied by  any  species  cannot  intercross  directly  unless  the  area 
be  very  limited  in  extent  or  their  powers  of  migration  very 
considerable.  And  even  the  birds,  whose  powers  of  migra- 


68  ORGANIC  EVOLUTION 

tion  are  so  well  known,  usually  breed  year  after  year  in  the 
same  localities,  the  same  individuals  returning  each  spring  to 
the  same  spot  and  often  occupying  the  same  nest  that  was 
left  the  year  before.  Of  course,  as  those  individuals  of  the 
species  which  occupy  the  intermediate  area  will  breed  freely 
with  those  nearer  the  two  extremes,  the  segregation  of  the 
latter  is  but  partial,  yet  it  must  be  sufficient  to  affect 
evolution. 

Natural  selection,  sexual  selection,  and  segregation  all 
mutually  interact,  as  we  can  readily  see.  Sexual  selection, 
the  exercise  of  choice  in  mating,  causes  reproductive  segrega- 
tion, and  this,  in  turn,  may  affect  natural  selection.  Let  me 
again  quote  Lloyd  Morgan  :  "  Among  the  wild  horses  in  Para- 
guay those  of  the  same  colour  and  size  associate  together; 
while  in  Circassia  there  are  three  races  of  horses  which  have 
received  special  names,  and  which,  when  living  a  free  life, 
almost  always  refuse  to  mingle  and  cross,  and  will  even 
attack  one  another.  In  one  of  the  Faroe  Islands,  not  more 
than  half  a  mile  in  diameter,  the  half-wild  native  black  sheep 
do  not  readily  mix  with  imported  white  sheep.  In  the  Forest 
of  Dean  and  in  the  New  Forest  the  dark  and  pale-coloured 
herds  of  fallow  deer  have  never  been  known  to  mingle; 
and  even  the  curious  ancon  sheep,  of  quite  modern  origin, 
have  been  observed  to  keep  together,  separating  themselves 
from  the  rest  of  the  flock  when  put  into  enclosures  with  other 
sheep.  .  .  .  This  preference  of  animals  for  their  like,  even 
in  the  case  of  slightly  different  varieties  of  the  same  species, 
is  evidently  a  fact  of  great  importance  in  considering  the 
origin  of  species  by  natural  selection,  since  it  shows  us  that, 
so  soon  as  a  slight  differentiation  of  form  or  colour  has  been 
effected,  isolation  will  at  once  arise  by  the  selective  action 


SEGREGATION  69 

of  the  animals  themselves."  This  is  a  good  statement  of  the 
case  except  that  Lloyd  Morgan  should  have  said  isolation 
may  at  once  arise,  not  "  will "  at  once  arise. 

Romanes l  has  called  attention  to  a  factor  in  segrega- 
tion which  has  as  yet  been  insufficiently  studied,  but  which 
may  prove  of  the  greatest  importance.  He  has  called  it 
physiological  selection.  It  has  been  observed  that  certain 
individual  animals  of  the  same  species,  when  crossed  with 
each  other,  are  infertile,  whereas  either  one,  if  crossed  with 
a  different  mate,  might  have  been  normally  fertile.  There 
exists  some  insufficiently  understood  bar  to  fertility  between 
those  two  individuals.  This  is  a  restraint  upon  the  perfect 
freedom  of  intercrossing,  a  sort  of  negative  segregation,  and 
must  have  a  real  effect  on  evolution.  It  seems  quite  pos- 
sible that  further  observation  and  experiment  may  show 
this  factor  in  segregation  to  be  more  common  and 'impor- 
tant than,  in  our  present  ignorance  of  the  actual  facts,  we 
can  assert.  The  reproductive  function  is  very  delicate  and 
liable  to  disturbance  from  apparently  slight  causes.  Many 
wild  animals,  however  well  kept,  are  barren  in  captivity  or 
are  less  fertile  than  when  unrestrained.  Transportation  to 
a  strange  locality,  sometimes  interferes  with  reproduction. 
Again,  there  are  some  observations  which  suggest  that 
variation  in  structure  in  any  of  the  different  organs  of  the 
body  may  be  correlated  with  such  disturbance  of  the  repro- 
ductive functions  as  to  decrease  the  fertility  of  crosses  be- 
tween the  individuals  which  diverge  from  the  species  type 
and  those  which  do  not  so  diverge.  This  point,  however, 
needs  much  more  study  before  we  can  determine  the  im- 
portance of  its  influence  in  producing  physiological  segre- 

1  Darwin  and  After  Darwin,  Volume  III,  "Isolation  and  Physiological  Selection." 


70  ORGANIC  EVOLUTION 

gation.  If  it  be  true  that  closely  related  individuals,  when 
bred  together,  are  more  fertile  than  are  distant  relatives, 
as  seems  under  some  circumstances  to  be  true,  this  fact 
also  is  of  great  importance.  The  whole  subject  of  physio- 
logical selection  needs  much  more  study.  It  is  surely  of 
some  importance  as  a  cause  of  segregation;  it  may  be  of 
great  importance. 

Segregation  might  cause  the  perpetuation  of  divergent 
characters,  though  these  were  of  no  use  and  so  not  subject 
to  the  preserving  action  of  natural  selection.  This,  how- 
ever, would  not  produce  adaptation  to  the  environment, 
which  is  the  striking  character  of  animals  and  plants. 
Segregation,  therefore,  unaided  by  natural  selection,  cannot 
have  been  an  important  factor  in  that  evolution  of  animals 
and  plants  which  we  find  has  taken  place,  bringing  them 
into  harmony  with  their  environment.  Segregation  becomes 
important  when  it  acts  in  connection  with  the  other  factors 
of  evolution,  natural  selection,  sexual  selection,  and,  possibly 
also,  among  lowly  forms,  in  connection  with  the  inheritance 
of  parental  modifications. 

THE   INHERITANCE    OF   PARENTAL   MODIFICATIONS 

One  more  factor  in  evolution  needs  careful  discussion, 
namely,  the  inheritance  of  parental  modifications,  that 
which  Weismann  has  called  the  "  inheritance  of  acquired 
characters."  For  this  factor  the  largest  claims  are  made 
by  some  biologists. 

It  is  well  known  that  both  animals  and  plants  change 
constantly  during  their  whole  lives  as  a  result  of  the  effects 
on  them  of  the  environment,  and  through  the  reaction  upon 


INHERITANCE   OF  PARENTAL   MODIFICATIONS        *Jl 

themselves  of  their  own  activity.  Use  strengthens  a  muscle 
and  disuse  allows  it  to  waste  away.  Some  claim  that,  as 
a  matter  of  course,  any  such  effect  produced  in  one  indi- 
vidual will  be  handed  down  to  his  descendants,  and  that 
here  we  have  a  most  potent  cause  of  evolution  in  the  trans- 
mission to  the  offspring  of  the  modifications  produced  in 
the  parent.  Favorable  or  poor  conditions  of  nutrition  pro- 
duce great  effects  on  individual  plants  and  animals;  so  also 
do  climatic  conditions.  Are  these  effects  upon  the  indi- 
viduals of  one  generation  transmitted  to  their  offspring  of 
the  next  generation  ?  If  so,  this  inheritance  of  parental 
modifications  must  have  the  greatest  influence  upon  evolu- 
tion. The  matter  needs  careful  scrutiny. 

Among  the  lower  forms  of  living  things,  the  unicellular 
forms,  many  of  which  are  so  lowly  that  we  cannot  determine 
whether  they  be  animals  or  plants,  among  these  lower  forms 
the  inheritance  of  parental  modifications  seems  at  first 
thought  altogether  probable,  as  a  single  illustration  will 
suffice  to  show.  An  Amoeba  is  a  lowly  animal  of  micro- 
scopic size,  consisting  of  a  bit  of  protoplasm  with  a  single 
nucleus.  It  has  no  highly  differentiated  organs,  but  the 
whole  body  takes  part  in  the  performance  of  each  function. 
When  this  animal  reproduces,  it  merely  divides  into  two  (or 
more)  little  Am&bce,  each  of  which  eats  and  grows  again  to 
the  characteristic  adult  size,  when  the  process  of  division  is 
repeated.  The  offspring  are  merely  parts  of  the  original 
parent,  and  of  course  show  in  themselves  the  features  of 
organization  characteristic  of  this  parent.  It  seems  possible 
that  modifications  of  the  parent  might  affect  the  offspring, 
which  are  but  detached  portions  of  the  parent.  Even  the 
effects  of  injuries  to  the  parent  might  be  inherited  by  the 


72  ORGANIC  EVOLUTION 

offspring.  Parental  modifications  among  the  unicellular  ani- 
mals and  plants  might,  then,  often  be  inherited.  The  whole 
body  divides,  leaving  no  residue,  so  that  any  modification  in 
the  parent  might,  one  would  say,  pass  directly  to  the  offspring. 

But  we  must  exercise  here  some  caution,  and  not  too 
readily  accept  first  impressions.  Not  only  in -higher  organ- 
isms, but  in  unicellular  forms  as  well,  it  seems  clearly  to  be 
true  that  physiological  activity  and  inheritance  is  controlled 
by  a  definite  portion  of  the  nucleus,  the  so-called  chromatin, 
and  if  any  quality  is  to  be  perpetuated  for  any  number  of 
generations  it  must  be  or  become  ingrained  in  the  very 
structure  of  this  part  of  the  nucleus.  The  inheritance  of 
parental  modifications  in  the  unicellular  animals  and  plants 
is  not  then  so  simple  a  matter  as  it  seems  at  first  sight. 

But  how  is  it  with  more  highly  organized  animals  in 
which  the  body  is  differentiated  into  different  portions,  each 
with  its  special  function,  —  bone,  muscle,  nerve,  digestive 
organs,  renal  organs,  and  a  great  number  more  of  special 
organs  and  tissues?  In  these  higher  animals,  and  in  the 
higher  plants  as  well,  the  function  of  reproduction  is  not  per- 
formed by  the  body  as  a  whole,  but  is  given  over  to  special 
groups  of  cells,  the  germ  cells,  constituting  the  ovaries  and 
testes.  It  is  these  cells,  and  these  only,  which  under  ordinary 
conditions  give  rise  to  new  individuals.  Under  such  circum- 
stances the  problem  of  the  inheritance  of  parental  modi- 
fications is  not  so  simple.  How  can  the  enlargement  of  a 
muscle,  due  to  exercise,  so  affect  the  germ  cells,  which  lie 
perhaps  at  a  distance  from  the  muscle  in  question,  as  to 
cause  the  new  individual,  which  shall  arise  from  these  germ 
cells,  to  have  the  corresponding  muscle  in  its  body  enlarged  ? 
The  question,  we  see,  is  not  a  simple  one. 

The  germ  cells  in   the  body   are    the  only  ones  which 


INHERITANCE    OF  PARENTAL  MODIFICATIONS       73 

under  ordinary  conditions  have  any  descendants  in  the  fol- 
lowing generation.  The  whole  body  of  the  offspring  comes 
from  two  united  germ  cells,  —  an  egg  from  one  parent  and  a 
spermatozoon  from  the  other  parent.  No  bone  cell,  or  muscle 
cell,  or  any  other  body  cell,  in  either  parent,  gives  rise  to  any 
part  of  the  offspring.  Weismann  has  used  the  term  soma  to 
include  all  the  cells  of  the  body  which  are  not  germ  cells, 
that  is,  the  muscle  cells,  bone  cells,  nerve  cells,  etc.  .  .  . 
This  distinction  between  the  germ  cells,  from  which  the 
young  are  derived,  and  the  soma  cells,  which  ordinarily  have 
no  offspring  in  the  next  generation  but  are  destined  to  die,  is 
a  very  important  one,  and  upon  it  must  be  based  the  discus- 
sion of  the  inheritance  of  parental  modifications. 

As  a  fertilized  egg  is  developing  into  an  adult  organism 
it  divides  into  a  number  of  portions  called  blastomeres, 
certain  of  which  will  form  the  germ  cells  of  the  new  organ- 
ism, while  the  remainder  will  become  its  soma.  The  germ 
cells  of  one  generation  are  thus  derived  almost  directly  from 
the  germ  cells  of  the  preceding  generation. 

The  accompanying  diagram  may  make  the  matter  clearer. 


Generation  A, 


Germ  cells 


Soma. 


Generation  B, 

Generation  C, 
Generation  D, 

In  the  diagram  the  lines  indicate  lines  of  descent.  Both  the 
germ  cells  and  soma  cells  of  any  generation  are  derived  from 
the  germ  cells  alone  of  the  preceding  generation.  The 


74  ORGANIC  EVOLUTION 

soma  cells  have  no  descendants.1  They  die  without  off- 
spring. Moreover,  apparently  no  germ  cell  has  ever  been 
anything  but  a  nascent  germ  cell.  It  has  never  been  a 
muscle  cell  or  a  nerve  cell.  Muscle  cells,  or  any  other  highly 
differentiated  soma  cells,  do  not  change  into  germ  cells. 

We  can  leave  out  of  account  the  processes  of  asexual 
reproduction  (fission,  budding,  reproduction  by  asexual 
spores),  for,  while  modifications  of  the  soma  of  the  parent 
could  pass  from  parent  to  offspring  by  these  processes  of 
asexual  reproduction,  the  modifications,  if  unable  to  be  inher- 
ited through  sexual  reproduction,  would  be  lost  whenever, 
perhaps  after  several  asexually  produced  generations,  sexual 
reproduction  should  intervene ;  and  we  know  of  no  species 
of  multicellular  animal,  or  higher  plant,  which  reproduces 
indefinitely  by  asexual  methods.  Sexual  reproduction  inter- 
venes sooner  or  later.  The  fact  that  asexual  reproduction 
occurs  does  not,  then,  alter  the  general  argument  in  regard 
to  the  inheritance  of  parental  modifications. 

The  phenomena  of  regeneration  would  be  of  some  inter- 
est in  this  connection,  if  we  knew  well-authenticated  instances 
of  animals  regenerating  their  reproductive  organs,  forming 
from  soma  cells  the  new  germ  cells  to  take  the  place  of  those 
which  had  been  lost.  We  do  not  know,  however,  that  such 
regeneration  is  customary,  or  even  possible,  in  any  group  of 
animals.  Certainly  it  is  not  of  sufficient  frequency  to  be 
taken  into  account  as  a  means  by  which  soma  cells  might 
impress  their  character  upon  germ  cells  and  thus  secure  the 
inheritance  of  parental  modifications. 

1  As  the  diagram  shows,  the  body  (soma)  of  the  "  parent "  and  the  body  (soma) 
of  the  '•  child  "  are  in  the  relation  of  uncle  and  nephew,  being  related  only  through, 
the  germ  cells  of  the  parent's  parents. 


INHERITANCE    OF  PARENTAL   MODIFICATIONS       75 

The  relation  of  soma  and  germ  cells  in  plants  and  the 
relation  of  the  germ  substance  to  the  processes  of  regenera- 
tion in  plants  are  more  obscure  than  the  similar  relations  in 
animals.  It  does  not  seem  best  to  attempt  to  discuss  them 
here. 

The  modifications  of  the  soma,  to  which  we  must  refer, 
are  of  two  sorts,  first,  those  produced  by  the  effect  of  the 
environment  upon  the  organism,  and,  second,  those  resulting 
from  the  reaction  upon  itself  of  the  activity  of  the  animal 
or  plant.  Let  us  illustrate  each. 

The  direct  influence  of  food  and  climate  is  often  of  such 
a  nature  as  to  produce  changes  in  the  individual.  For  exam- 
ple, plants,  if  grown  in  a  warm  moist  climate  and  in  rich  soil, 
may  be  larger  than  if  grown  under  less  favorable  circum- 
stances. Will  these  plants  have  larger  offspring  as  a  result 
of  inheritance  of  the  increased  size  ?  Is  the  direct  effect 
of  the  favorable  environment  (increased  size)  handed  down 
to  the  offspring  ?  If  the  offspring  be  large,  as  they  probably 
will  be,  is  their  large  size  due  to  the  fact  that  their  parents 
became  large  under  the  favorable  conditions  in  the  midst  of 
which  they  grew,  or  to  the  fact  that  the  offspring  themselves 
grow  under  the  same  favorable  conditions  as  their  parents 
and  so,  for  this  reason,  are  large  ?  That  is,  is  their  size 
due  to  the  inheritance  of  the  increased  size  of  their 
parents,  or  to  the  same  favorable  soil  and  climate  that  made 
their  parents  large  ?  Is  there  at  all  any  inheritance  of 
increased  size  ?  How  can  we  tell  ?  We  have  at  least  one 
test  which  we  may  apply.  When  plants  are  taken  from 
unfavorable  conditions  and  are  grown  under  the  most  favor- 
able conditions,  do  they  only  gradually  assume  increased  size, 
or  are  those  of  the  first  or  second  generation  as  large  as 


76  ORGANIC  EVOLUTION 

those  of  the  third  or  fourth  or  tenth  or  fiftieth  ?  If  parental 
modifications  be  inherited,  the  plants  of  the  later  generations 
should  be  larger  than  those  of  the  first,  the  inherited  effect 
of  increased  size  accumulating  from  generation  to  genera- 
tion. We  do  not,  however,  find  this  to  be  the  case.  It  is 
not  by  this  method  that  large  plants  have  been  produced 
by  the  gardeners.  They  have  been  produced  by  selecting 
the  larger  plants  to  breed  from  and  continuing  this  process 
from  generation  to  generation,  the  same  process  of  selection 
that  goes  on  in  nature. 

Let  us  look  at  an  illustration  of  the  reputed  inheritance 
of  the  effects  of  use  and  disuse  and  see  if  we  can  accept  this 
influence  as  a  factor  in  the  evolution  of  the  higher  animals 
and  plants.  We  have  referred  to  the  increase  in  size  that 
follows  the  use  of  a  muscle,  and  the  decreased  size  that 
results  from  its  disuse.  Are  these  effects  inherited  by  the 
offspring  ?  Does  the  man  who  is  strong  because  he  leads 
an  active  life  have  stronger  children  than  he  would  have 
if  he  led  an  inactive  life?  Notice  this:  The  fact  that  he 
does  develop  strong  muscles  as  the  result  of  exercise  shows 
that  he  must  have  had  an  innate  capacity  for  developing 
strong  muscles  by  exercise.  If  he  inherited  from  his  parents 
the  ability  to  develop  strong  muscles  under  the  stimulus  of 
an  active  life,  his  offspring  in  turn  will  inherit  from  him  the 
same  ability.  A  blacksmith  has  a  son  who  becomes  an  office 
clerk  and  takes  no  exercise.  Does  the  son  have  any  stronger 
right  arm  than  he  would  have  had  if  his  father  had  been 
an  office  clerk  ?  Of  course  the  son  will  have  the  same 
capacity  for  developing  a  strong  arm  that  his  father  had 
before  him,  but  will  the  fact  that  the  father  developed  this 
capacity  and  became  strong  give  the  son  any  greater  strength 


INHERITANCE    OF  PARENTAL  MODIFICATIONS       77 

than  he  would  have  had  if  the  father,  through  inactivity, 
had  allowed  his  capacity  for  strength  to  lie  undeveloped  ? 
There  is  little,  if  any,  carefully  scrutinized  and  carefully 
recorded  evidence  in  favor  of  an  affirmative  answer. 

How  can  we  test  the  case  ?  It  is  very  difficult.  Experi- 
mentation has  failed  to  show  inheritance  of  the  effects  of  use 
and  disuse  among  the  higher  plants  and  animals,  and  we 
have  practically  no  evidence  in  its  favor  except  its  apparent 
plausibility.  But,  when  carefully  scrutinized,  is  it  as  plausible 
as  it  seems  at  first  thought  ?  How  can  the  use  of  the  biceps 
muscle  in  the  arm  of  the  parent  so  affect  the  offspring  that 
he  will  be  not  only  stronger,  but  stronger  in  the  biceps 
muscle,  the  particular  part  affected  in  the  parent  ?  The  child 
is  not  the  child  of  the  biceps  muscle  of  the  parent,  but  the 
child  of  the  germ  cells  of  the  parent,  and  these  germ  cells 
have  little  to  do  with  the  parent's  biceps  muscle.  They  are 
separated  by  a  great  space,  and  they  do  not,  so  far  as  we 
know,  have  any  special  mutual  relation.  If  the  increased 
strength  gained  by  the  biceps  muscle  of  the  parent  is  to  be 
handed  down  to  the  offspring,  the  increase  in  size  in  the 
parent's  biceps  must  in  some  way  produce  an  effect  upon 
the  parent's  distant  germ  cells  from  which  the  child  is  to 
develop ;  and  this  effect  upon  the  germ  cells  must  be  of  so 
particular  and  definite  a  kind  as  to  produce  not  a  general 
effect  upon  the  offspring  but  a  particular  effect,  namely, 
greater  strength,  and  not  only  greater  strength,  but  greater 
strength  in  a  particular  portion  of  the  body,  the  biceps  muscle 
of  the  right  arm.  Th^  hypothesis,  apparently  so  simple  at 
first  glance,  is  seen,  wh^n  scrutinized,  to  involve  a  connection 
between  muscle  cells  and  the  distant  germ  cells  so  intimate 
and  so  definite  as  to  be  marvellous  beyond  almost  any  known 


78  ORGANIC  EVOLUTION 

fact  of  biology.  No  greater  assumption  has  ever  been  made 
as  the  basis  of  any  biological  theory,  and  it  is  pure  assump- 
tion, for  as  yet  we  have  no  evidence  of  any  such  mechanism 
connecting  muscle  or  nerve  or  bone  cells  with  the  germ  cells. 
In  the.  absence  of  evidence  in  favor  of  the  inheritance  of 
parental  modifications  among  highly  organized  forms,  and  in 
the  presence  of  the  tremendous  assumption  upon  which  this 
hypothesis  rests,  I  think  it  unsafe  to  accept  this  principle 
even  as  a  working  theory.  We  may  get  definite  evidence 
sometime  that  will  lead  us  to  a  different  conclusion.  The 
phenomena  of  biology  are  wonderful,  and  even  this  great 
assumption  may  yet  be  proven.  It  has  not  yet  been  proven 
or  been  shown  to  be  probable. 

Let  us  direct  our  attention  to  two  further  points  in  con- 
nection with  this  part  of  the  discussion.  Many  of  the  most 
remarkable  phenomena  of  nature  we  are  sure  have  been 
developed  without  the  aid  of  the  inheritance  of  parental 
modifications,  so  we  do  not  need  the  help  of  this  hypothesis 
because  natural  phenomena  are  "  too  wonderful  to  be  ex- 
plained without  it."  The  color  of  flowers  is  useful  to  attract 
insects.  It  has  served  its  purpose  when  an  insect  has  seen 
the  color  and  has  responded.  The  plant  lies  passive ;  the 
insect  actively  responds.  How  can  the  reactionary  effect  of 
the  active  response  in  the  insect  be  inherited  by  the  offspring 
of  the  plant  ?  Or  another  equally  absurd  case :  Many 
animals,  rabbits  for  example,  are  protectively  colored.  This 
protective  color  serves  its  purpose,  i.e.  is  used,  when  the  fox 
fails  to  see  the  rabbit.  How  can  the  failure  of  the  fox  to  see 
the  rabbit  produce  such  an  effect  on  the  germ  cells  of  the 
rabbit  that  the  offspring  of  the  rabbit  shall  be  still  more  pro- 
tectively colored  ?  Again  :  many  seeds  have  spines  or  hooks 


INHERITANCE    OF  PARENTAL  MODIFICATIONS      79 

on  their  outer  surfaces,  which  become  entangled  in  the 
wool  of  animals  or  the  clothing  of  men,  and  so  secure  the 
scattering  of  the  seeds  at  a  distance.  These  hooks  dry  up 
by  the  time  the  seeds  are  ripe,  and  are  nothing  but  dead  hard 
tissue  incapable  of  receiving  any  impression.  They  cannot, 
then,  hand  down  the  effects  of  their  use  to  the  next  genera- 
tion. This  is  all  the  more  true,  since,  at  the  time  of  their 
use,  they  are  separated  from  the  plant  of  which  they  were  a 
part,  and  so,  of  course,  can  have  no  effect  on  the  germ  cells 
of  that  plant.  Of  course,  the  dry  seed  coats  can  have  no 
vital  relation  to  the  little  embryo  they  enclose. 

Again,  the  instincts  of  the  bees,  to  which  we  have  already 
referred,  are  wonderful.  The  worker-bees,  which  are  the 
ones  with  the  remarkable  instincts,  build  the  honeycomb, 
gather  and  store  the  honey,  feed  the  young,  control  the 
queen,  manage  the  whole  hive  in  fact,  with  an  intelligence, 
or  in  accordance  with  instincts,  of  the  highest  order.  It  is 
the  workers  alone  who  have  these  wonderful  instincts,  but  the 
workers  are  practically  sterile,  very  rarely  having  offspring; 
so,  apparently,  the  instincts  of  the  workers  cannot  have  been 
developed  through  the  inheritance  of  the  effects  of  use.  The 
workers  have  no  offspring  to  whom  they  could  hand  down 
their  instincts.  The  workers  come  from  eggs  laid  by  the 
queen,  and  it  seems  to  have  been  natural  selection,  choosing 
for  survival  those  hives  in  which  the  workers  are  most  intel- 
ligent, or  have  the  most  perfect  instincts,  that  has  produced 
the  complex  activities  of  the  present  beehive.  This  has 
been  urged  by  Weismann  and  others  as  an  example  of  great 
development  of  instinct  or  intelligence  which  natural  selec- 
tion alone  can  have  produced. 

Now,  while  I  believe  that  the  remarkable  instincts  of  the 


8o  ORGANIC  EVOLUTION 

worker-bees  have  been  developed  through  natural  selection, 
I  would  suggest  that  the  argument  stated  above  is  hardly 
conclusive.  The  sterility  of  the  worker-bees  is  a  character 
acquired  within  comparatively  recent  times.  Their  compli- 
cated instincts  (or  high  degree  of  intelligence)  may  have 
been  acquired  before  they  became  sterile.  This  possibility 
is  suggested  by  the  fact  that  the  fertile  females  of  certain 
wasps  have  most  remarkable  instincts,  almost,  if  not  fully, 
as  wonderful  as  those  of  the  worker-bees.  Among  the 
solitary  wasps  the  fertile  females  never  cease  to  exercise 
their  special  instincts.  Among  some  of  the  social  wasps, 
on  the  other  hand,  we  find  species  in  which  the  fertile 
females  exercise  these  instincts  for  a  time  and  later  cease 
to  use  them.  Dr.  and  Mrs.  Peckham  say  of  the  genera 
Vespa  and  Polistes:  "In  the  autumn  the  queens,  having 
mated  with  the  drones,  creep  away  into  crevices  and  shel- 
tered corners,  where  they  pass  the  winter.  In  the  spring 
they  may  be  seen  seeking  for  suitable  nesting  places,  and 
forming,  from  the  fibres  of  weather-beaten  wood,  which  are 
scraped  off  and  chewed  up,  the  first  layer  of  cells.  So 
much  being  accomplished,  the  queen  deposits  her  eggs,  one 
in  each  cell,  and  when  these  develop  into  grubs  she  feeds 
them,  until  at  the  end  of  a  week  or  ten  days  they  spin  their 
cocoons  and  become  pupae.  In  from  eight  to  ten  days  the 
perfect  wasp  is  formed  and  emerges  from  its  cell  ready  to 
assume  its  share  of  responsibility  in  the  work  of  the  nest. 
These  first  wasps  are  always  neuters,  and  hereafter  all  the 
duties  which  the  queen  has  been  obliged  to  perform,  with  the 
single  exception  of  egg-laying,  fall  upon  them."  The  neuters 
of  these  social  wasps  die  when  winter  comes  on.  Should 
they  live  through  the  winter,  there  would  be  no  need  of  the 


INHERITANCE    OF  PARENTAL  MODIFICATIONS      8 1 

fertile  females  retaining  their  special  instincts  of  nest-build- 
ing and  caring  for  the  young.  These  activities  might  then 
be  left  wholly  to  the  neuter  workers,  which  would  give  us 
the  condition  found  among  the  bees  at  present.  It  seems 
not  improbable  that  this  has  been  the  general  course  of  the 
development  of  the  instincts  of  the  worker-bees.  I  have 
given  Weismann's  argument  because  it  is  one  so  often 
quoted,  though  it  is  not  conclusive.  There  are,  however, 
many  classes  of  phenomena  whose  development  can  be  ex- 
plained by  natural  selection  but  not  by  the  inheritance  of 
parental  modifications,  and  these  phenomena  are  as  remark- 
able as  any  we  have  to  explain.  We  do  not  need  the  hypoth- 
esis of  the  inheritance  of  parental  modifications  to  explain 
nature  because  of  natural  phenomena  being  "  too  wonderful 
for  any  other  explanation." 

Finally,  the  inheritance  of  parental  modifications,  even 
if  it  occurred,  would  be  wholly  inadequate  to  explain  the 
most  fundamental  feature  of  the  phenomena  of  organic 
nature ;  namely,  the  adaptation  of  the  organism  to  its 
environment.  Adaptation  is  the  key-note  of  organic  nature, 
and  it  is  exactly  the  thing  natural  selection  secures,  for  those 
individuals  which  are  not  adapted  to  their  environment  are 
destroyed  in  the  stmiggle  for  existence,  leaving  only  the  well- 
adapted  forms  alive.  The  inheritance  of  parental  modifica- 
tions, on  the  other  hand,  could  not  produce  adaptation  to  the 
environment,  unless  the  influence  of  the  environment  upon 
each  individual  organism  and  the  reaction  of  the  organism 
itself  were  such  as  to  produce  adaptation  of  each  individual 
to  -its  environment,  and  we  are  far  from  having  sufficient 
evidence  that  the  direct  changes  produced  in  each  individual 
by  the  influence  of  the  environment  are  thus  adaptive.  For 


82  ORGANIC  EVOLUTION 

example,  animals  living  in  cold  countries  have  thicker  fur 
than  tropical  species.  This  might  readily  be  brought  about 
by  natural  selection,  but  we  have  little  to  indicate  that  the 
direct  effect  of  cold  upon  each  individual  is  such  as  to  cause 
increased  thickness  of  hair. 

One  more  question  naturally  presents  itself.  If  changes 
in  the  offspring  are  not  produced  by  changes  in  the  body 
(soma)  of  the  parent,  how  do  variations  come  to  appear  in 
the  offspring?  Variations  arise  in  the  germ  cells  and  are 
transmitted  from  them  to  their  offspring.  Changes  in  the 
internal  constitution  of  the  germ  cells  will  cause  changes  to 
appear  in  the  young  which  arise  from  these  germ  cells.  The 
character  of  every  animal  or  plant  is  dependent  upon  the 
character  of  the  germ  cell  from  which  it  comes.1  A  new- 
laid  egg  of  a  chicken  almost  exactly  resembles  a  new-laid  egg 
of  a  duck.  The  most  careful  study  of  the  two  would  not 
show  any  trace  of  the  differences  which  are  to  appear  as  the 
eggs  develop ;  yet  it  must  be  that  the  two  eggs  differ  in  their 
constitution  and  that  to  this  difference  in  structure  is  due  the 
difference  between  the  birds  which  will  hatch  from  the  two 
eggs.  The  character  of  the  adult  is  predetermined  by  the 
character  of  the  egg.  Of  course,  then,  anything  which 
causes  changes  in  the  character  of  the  egg  may  cause 
correlated  changes  in  the  adult  which  is  developed  from 
the  egg. 

But  what  can  cause  such  changes  in  the  egg  or  spermato- 
zoon ?  It  lies  inside  the  body  of  the  animal  or  plant  which 

1  This  is  equally  true  whether  we  believe  with  Weismann  that  every  organ  of  the 
future  adult  is  represented  by  a  corresponding  differentiated  though  minute  particle 
in  the  germ,  or  with  Hertwig  that  the  germ  cell  is  more  nearly  homogeneous,  differ- 
entiation appearing  as  growth  proceeds. 


INHERITANCE    OF  PARENTAL   MODIFICATIONS       83 

bears  it,  and  is  to  a  considerable  degree  protected  from  con- 
tact with  the  outer  world.  Why,  then,  does  the  egg  change 
its  constitution  ? 

Those  who  are  at  all  familiar  with  biological  phenomena 
know  that  all  living  things  and  all  parts  of  their  bodies  are 
constantly  changing.  No  bit  of  living  protoplasm  is  ever 
at  rest.  It  liberates  the  energy  used  in  its  different  life 
activities  only  by  the  destruction  of  some  of  its  sub- 
stance, and  this  constant  waste  has  to  be  constantly 
repaired.  For  this  repair  food  is  needed  and  is  digested  and 
assimilated,  being  built  up  into  new  protoplasm  to  take  the 
place  of  that  which  was  destroyed.  Changes  in  nutrition 
may  cause  changes  in  the  constitution  of  the  organism  which 
is  being  nourished.  The  constitution  of  the  germ  cells  may 
thus  vary  with  the  changing  conditions  of  nutrition,  and  such 
changes  in  the  structure  of  the  germ  cells  may  register  them- 
selves in  changes  in  the  organisms  which  arise  from  these 
germ  cells.  Variation  in  animals  and  plants  may  therefore 
be  due  to  the  conditions  of  nutrition  of  the  germ  cells  from 
which  they  came. 

Germ  cells  receive  their  nutriment  from  the  blood  or 
lymph  in  all  higher  animals.  The  blood  may  contain  other 
substances  than  food  which  will  affect  the  character  of  the 
germ  cells.  Changes  in  the  blood  other  than  those  con- 
nected with  nutrition  may  therefore  cause  changes  in  the 
germ  cells,  producing  variation  in  the  offspring.  Such 
changes  in  the  constitution  of  the  blood  may  be  due  to 
the  action  of  the  somatic  cells,  since  their  waste  products 
and  secretions  find  their  way  into  the  blood.  One  can 
readily  conceive,  for  example,  that  imperfect  action  of  the 
renal  cells  (perhaps  due  to  disease),  resulting  in  impure 


84  ORGANIC  EVOLUTION 

blood,  might  so  affect  the  germ  cells  as  to  cause  the  offspring 
which  arise  from  them  to  diverge  somewhat  from  the  usual 
character.  It  is  hard  to  see  how  this  somewhat  indefinite 
effect  of  soma  upon  germ  could  be  avoided.  We  have,  how- 
ever, no  evidence  that  the  substances  given  off  by  the  several 
sorts  of  soma  cells  into  the  blood  affect  the  germ  cells  in 
such  a  way  that  when  they  give  rise  to  new  organisms  these 
will  repeat  in  their  own  bodies  those  peculiar  modified 
somatic  activities  of  their  parents  which  gave  into  the  blood 
the  substances  which  caused  the  modification  of  the  germ 
cells.  So,  while  we  recognize  the  probability  that  germ  cells 
are  constantly  affected  by  changes  in  the  blood  due  to  the 
activity  of  soma  cells,  and  while  recognizing  also  that  we  may 
have  here  a  real  cause  of  variation,  we  still  have  no  evidence 
that  these  somatic  influences  upon  the  germ  are  of  such  a 
nature  as  to  cause  the  offspring  to  inherit  the  adventitious, 
accidental,  or  secondarily  acquired  somatic  characters  of  the 
parent.  We  have  here  a  -probable  cause  of  variation,  but 
not  a  means  for  securing  the  inheritance  in  kind  of  modifica- 
tions of  the  parental  soma. 

Again  observe  that  when  a  spermatozoon  unites  with  an 
egg  in  the  process  of  fertilization,  there  are  mingled  germ 
cells  from  two  different  ancestors,  each  with  its  own  he- 
reditary potentialities.  The  organism  resulting  from  the 
development  of  this  compound  cell  will  naturally  be  dif- 
ferent from  either  of  its  parents,  the  hereditary  tendencies 
received  from  one  parent  being  modified  by  those  from  the 
other  parent.  For  a  proper  understanding  of  the  possi- 
bilities of  variation  which  are  involved  in  this  fact  of  the 
union  of  two  germ  cells  in  the  process  of  fertilization  one 
needs  to  be  familiar  with  some  of  the  most  intricate 


SUMMARY  85 

phenomena  of  cell  structure  and  physiology,  which  it  is  not 
appropriate  to  describe  here. 

In  closing  this  exposition  of  the  theory  of  organic  evolu- 
tion it  is  well  to  call  attention  to  one  important  point.  The 
whole  process  of  evolution  centres  in  the  processes  of  repro- 
duction. Natural  selection  is  the  selection  of  the  individuals 
who  are  to  perpetuate  the  species,  and  not  merely  of  the  indi- 
viduals who  are  to  live  out  their  own  lives.  Sexual  selection  is 
the  selection  of  mates  in  breeding.  Segregation  is  the  pre- 
vention of  free  intercrossing  in  the  breeding  processes. 
Orthogenesis  shows  its  results  increasingly  through  many 
generations.  Parental  modifications  could  produce  an  effect 
upon  the  evolution  of  the  species  only  when  they  are  handed 
down  by  reproduction  to  the  following  generations.  The 
offspring  of  the  next  generation,  and  not  the  adults  of  the 
present  generation,  are  the  goal  in  all  the  processes  of  evolu- 
tion. Much  inaccurate  thinking  has  been  due  to  the  failure 
to  clearly  grasp  this  fundamental  conception.  Lloyd  Morgan 
sums  the  matter  up  in  the  phrase, "  To  breed  or  not  to  breed. 
That's  the  question." 

SUMMARY 

In  the  foregoing  rapid  review  we  have  noted  the  manner  of  operation 
of  these 

Factors  of  evolution : 
Natural  selection : 

Heredity  (Offspring  tend  to  resemble  their  parents)  : 
Variation  (This  resemblance  is  far  from  exact)  ; 
Variations  are  of  two  types 

( i )    Fluctuating  or  unstable  variations  furnishing  no  basis  for 
evolution  because  they  are  not  transmitted  to  offspring ; 


86  ORGANIC  EVOLUTION 

(2)    Stable  variations  or  mutations,  which  are  the  basis  of 

evolution  : 

The  destruction,  in  the  struggle  for  existence,  of  the  individuals 
which  are  not  adapted  to  their  environment,  resulting  in  a 
more  and  more  perfect  adjustment  of  organisms  to  the  con- 
ditions in  the  midst  of  which  they  have  to  live. 
Orthogenesis  : 

An  inherent  tendency  in  organisms  to  evolve  in  certain  direc- 
tions.    The  origin  of  such  tendencies  is  not  understood. 
Sexual  selection : 

The  exercise  of  choice  in  mating,  observed  among  spiders, 
insects,  and  vertebrates.  It  results  in  the  developing  of 
courting  habits,  of  conspicuous  colors,  ornamental  append- 
ages, beauty  (?)  of  voice,  etc.,  which  tend  to  make  the 
individuals  of  one  sex  (usually  the  males)  attractive  to  those 
of  the  other  sex. 
Segregation  : 

By  which  the  individuals  of  a  species  are  divided  into  different 
groups  which  do  not  freely  interbreed.     The  causes  of  seg- 
regation are  various  :    geographical,  climatic,  physiological, 
aesthetic,  etc. 
Inheritance  of  parental  modifications  : 

This  is  probably  not  an  efficient  cause  of  evolution  among 
unicellular  organisms,  and  apparently  is  never  effective 
among  higher  animals  and  plants. 

And  we  have  seen  that  all  of  the  processes  of  evolution  necessarily  centre 
in  reproduction. 


PART    SECOND 


II.     THE    PHENOMENA    EXPLAINED     BY    THE 
THEORY 

WE  have  reversed  the  natural  order  in  our  treatment 
of  the  theory  of  evolution.  It  was  the  phenomena,  to  which 
we  wish  now  to  direct  our  attention,  which  first  suggested 
the  theory,  and  it  was  only  by  prolonged  study  of  these 
phenomena  that  the  theory  was  tested  and  established. 
For  the  sake  of  brevity  in  the  presentation  of  the  subject, 
we  have  chosen  first  to  develop  the  theory  and  then  to 
apply  it  to  the  phenomena  upon  which  it  bears. 

For  the  purposes  of  our  treatment  the  phenomena  to 
which  we  wish  to  direct  attention  may  be  classified  as 
follows :  the  phenomena  of  comparative  anatomy ;  the  phe- 
nomena of  comparative  embryology ;  the  phenomena  of 
paleontology ;  the  phenomena  of  geographical  distribution ; 
and  the  phenomena  of  color  in  animals  and  in  the  blos- 
soms of  plants.  A  complete  discussion  of  these  subjects 
would  still  be  but  a  partial  treatment  of  the  phenomena 
which  have  a  bearing  upon  the  theory.  Many  points  of 
physiology,  the  phenomena  of  sterility,  hybridization,  in- 
stinct, habit,  etc.,  etc.,  would  still  be  omitted.  We  shall 
attempt  but  a  very  brief  treatment  of  some  of  the  phe- 
nomena of  the  several  types  mentioned  in  the  classification 
given  above.  Do  not,  then,  be  under  the  impression  that 
u.Te  shall  have  reviewed,  even  in  outline,  the  whole  subject. 

89 


9o 


ORGANIC  EVOLUTION 


The  phenomena  of  comparative  anatomy  in  their  bearing 
upon  the  theory  of  evolution. 

C las  si  fie  a  lion . 

AH  are  familiar  with  the  fact  that  animals  and  plants  are 
of  very  many  different  sorts,  and  that  the  different  kinds  show 

very  different  degrees  of 
complexity  in  their  or- 
ganization. We  give  ex- 
pression to  these  facts  in 
our  classification  of  ani- 
mals and  plants.  Forms 
which  are  closely  similar 
almost  to  the  point  of 
identity  we  call  members 
of  the  same  species.  For 
example,  while  hardly 
any,  if  any,  two  robins 
are  so  similar  that  we 
cannot  detect  some  dif- 
ferences between  them, 
still  all  robins  quite 
closely  conform  to  the 
same  type,  and  their  mutual  differences  are  so  slight  that 
without  hesitation  we  group  them  together  in  one  species. 
We  see  the  same  thing  among  plants.  Such  of  our 
common  blue  violets  as  have  rounded,  heart-shaped,  slightly 
pointed  leaves,  and  scentless  blue  flowers  of  large  size, 
having  also  very  much  shortened  stems,  we  class  under 
the  one  species  Viola  cucullata  (Fig.  n).  (There  are  other 
characters  of  the  species  besides  those  mentioned  by  which 


FIG.  ii. —  Viola  cucullata.  —  From  Britton  and 
Brown's  Illustrated  Flora  of  the  Northern  States  and 
Canada,  by  the  courtesy  of  the  authors  and  of  Charles 
Scribner's  Sons. 


COMPARATIVE   ANATOMY 


it  can  be  recognized.)  But  we  have  other  plants  whose 
blossoms  in  their  form  so  closely  resemble  those  of  the 
common  Viola  cucullata,  and  which  in  their  whole  appear- 
ance are  so  similar,  that  we  conclude  they  are  connected 
with  this  species;  yet  the  differences  are  sufficiently  great 
for  us  to  be  unable  to  assign  them  to  this  species.  One 
kind  of  these  violets  have 
smaller  blossoms  with  a 
much  longer  spur.  Their 
stems  are  highly  developed 
and  branching,  while  their 
leaves  are  smaller  and  are 
borne  upon  shorter  petioles 
(Fig.  12).  These  we  classify 
as  Viola  rostrata,  indicating 
the  difference  between  the 
two  types  by  the  different 
specific  names,  but  at  the 
same  time  calling  attention 
to  the  resemblance  between 
the  two  forms  by  giving 
them  both  the  same  genus 
name,  Viola.  There  are 
a  dozen  or  more  species  of  the  genus  Viola  found  around 
Baltimore.  In  this  same  region  is  found  an  apparently 
very  different  plant  with  tall  and  branching  stems,  with 
coarse  leaves  and  small  greenish  blossoms,  a  coarse,  weed- 
like  plant  (Fig.  13).  This  form  has  been  named  Solea 
concolor.  Now,  great  as  are  the  superficial  differences  be- 
tween this  species  and  our  violets,  careful  study  shows  that 
the  blossoms  of  both  are  made  up  on  the  same  plan,  and 


FIG.  12.  —  Viola  rostrata. —  From  Britton  and 
Brown's  Illustrated  Flora  of  the  Northern  States 
and  Canada,  by  the  courtesy  of  the  authors  and 
of  Charles  Scribner's  Sons. 


92 


ORGANIC  EVOLUTION 


that  there  are-  important  fundamental  resemblances  between 
Solea  and  the  members  of  the  genus  Viola.  This  funda- 
mental resemblance  in  the  midst  of  more  superficial  differ- 
ences we  indicate  by  classifying  both  Solea  and  Viola  in 
a  common  larger  group  which  we  call  the  family,  in  this 
case  the  violet  family  or  the  Violacetz.  As  we  have  sev- 
eral genera  within  the  one  family  Violacecz,  so  we  have 

many  different  families  of 
plants,  —  the  daisy  family 
or  Composite^  the  prim- 
rose family  or  Primulacea, 
the  rose  family  or  Rosacetz, 
and  so  on.  Now  all  these 
families  mentioned  have 
certain  general  resem- 
blances to  one  another, 
such  as  the  presence  of 
blossoms  and  seeds.  Many 
other  kinds  of  plants  are 
without  either  blossoms  or 
seeds ;  ferns  and  mosses, 
for  example.  We  distin- 
guish the  former  as  flowering  plants  or  phanerogams,  and 
the  latter  as  flowerless  plants  or  cryptogams.  Thus  we 
have  different  grades  in  the  classification  to  indicate  dif- 
ferent degrees  of  resemblance  and  divergence. 

Moreover,  as  we  study  the  different  groups  of  plants,  we 
find  them  very  different  in  the  complexity  of  their  organ- 
ization, in  the  extent  to  which  their  organs  and  tissues  are 
developed.  Some,  like  the  flowering  plants,  are  highly 
organized,  showing  very  elaborate  structure,  while  others 


FIG.  13.  —  Solea  concolor.  —  From  An  Illustrated 
Flora  of  the  Northern  States  and  Canada,  by  the  cour- 
tesy of  the  authors  and  of  Charles  Scribner's  Sons. 


COMPARATIVE  ANATOMY  93 

of  the  lower,  flowerless  plants,  such  as  the  yeast  plant,  or 
the  Algce,  are  very  simple  in  comparison.  In  the  same 
way,  among  animals  we  find  the  lowly  organized  Amceba. 
and  its  protozoan  relatives,  the  more  complex  sponges  and 
jellyfishes,  the  still  more  developed  flatworms,  the  annulated 
worms,  the  Crustacea,  the  spiders,  the  insects,  the  Mollusca 
(snails,  clams,  oysters,  etc.),  the  starfishes,  and  the  verte- 
brates, including  the  fishes,  Amphibia,  lizards,  birds,  and 
mammals. 

Now,  what  is  the  meaning  of  all  this  diversity  of  form 
and  the  various  degrees  of  complexity  ?  It  is  the  theory  of 
evolution  which  interprets  these  phenomena,  showing  us 
that  the  different  degrees  of  resemblance  and  divergence 
between  these  forms  indicate  different  degrees  of  relationship. 
Descent  from  common  ancestors,  with  divergence  under 
the  influence  of  natural  selection  and  the  other  factors  of 
evolution,  is  the  key  to  these  phenomena.  The  taxonomic 
system,  or  the  system  of  classification  of  animals  and  plants 
into  varieties,  species,  genera,  families,  orders,  subclasses, 
classes,  subkingdoms,  and  kingdoms,  is  but  an  expression 
of  relationships,  the  erection  of  a  genealogical  tree,  in  which 
the  animal  and  plant  kingdoms  would  be  the  two  great 
branches,  the  lesser  subdivisions  corresponding  to  the 
smaller  branches  and  the  twigs.  The  several  species  of 
violets  resemble  one  another  because  they  are  the  descend- 
ants of  common  ancestors,  and  that  is  what  we  mean  when 
we  class  them  in  the  same  genus  Viola.  Viola  and  Solea 
in  turn  have  a  still  more  remote  common  ancestor,  a  fact 
we  express  by  placing  the  two  genera  in  the  same  family, 
the  Violacea.  At  some  very  much  more  remote  period 
the  flowering  plants  were  derived  from  the  flowerless  plants, 


94 


ORGANIC  EVOLUTION 


and  we  give  expression  to  this  fact  when  we  establish  the 
two  major  divisions  of  the  plant  kingdom ;  namely,  Pkanero- 
gamia  and  Cryplogamia.  These  phenomena  of  taxonomy 
or  classification  were  unintelligible  until  the  theory  of  evolu- 
tion gave  us  the  talismanic  word  relationship. 

Homology. 

There    are    other   phenomena    of    comparative   anatomy 
fully  as  important    to    the  student  of  evolution.     The  phe- 


FlG.  14.  —  Skeletons  of  fore  limbs  of  various  vertebrates, 
a.  Wing  of  a  bird.     b.  Fore  leg  of  a  dog.    c.  Arm  of  man.    d.  Wing  of  bat. 

nomena  of  homology  are  of  great  interest.  The  wing  of  a 
butterfly  and  that  of  a  bird  serve  the  same  purpose  and 
are  built  on  the  same  mechanical  principle,  but  they  are 
fundamentally  different  in  their  structure.  On  the  other 
hand,  the  wing  of  a  bird  and  the  fore  leg  of  a  clog,  wrhile 
used  for  very  different  purposes  and  appearing  superficially 
to  be  very  different,  are  in  reality  very  much  alike  in  their 
fundamental  structure  (Fig.  14).  Each  has  four  chief  divi- 
sions,—  upper  arm,  fore  arm,  wrist,  and  hand,  —  and  in  each 
we  find  the  same  bones,  except  that  the  number  of  fingers  has 


COMPARATIVE   ANATOMY  95 

been  reduced  in  the  bird's  hand.  We  find  the  explanation 
of  this  resemblance  when  we  recognize  that  the  bird  and 
the  dog  are  descended  from  common  ancestors  in  which  the 
leg  was  used  for  walking ;  that  the  dog  has  perfected  the 
limb  for  walking,  while  the  bird  has  modified  and  adapted  it 
for  the  very  different  use,  flying.  The  two  organs  are  funda- 
mentally alike  because  they  are  modifications  of  the  same 
thing.  They  are  superficially  different  because  they  are 
used  for  very  different  purposes.  This  fundamental  re- 
semblance founded  on  common  descent  is  called  homology, 
and  the  phenomena  of  homology,  no  less  than  those  of 
taxonomy,  lend  much  support  to  the  evolution  theory,  being 
intelligible  in  the  light  of  that  theory,  while  without  this 
theory  they  have  no  meaning  to  us.  We  might  multiply 
almost  indefinitely  illustrations  of  homology  based  on  ge- 
netic relationship ;  the  illustration  given,  however,  will 
show  the  line  of  evidence  as  well  as  is  needed  for  our 
purpose. 

Vestigial  structures. 

Among  the  most  interesting  of  the  anatomical  evi- 
dences of  evolution  are  the  vestigial  organs  found  in  so 
many  animals  and  plants,  organs  once  normally  developed 
and  functional,  but  now  reduced,  and,  so  far  as  we  can 
judge,  functionally  insignificant.  Certain  snakes  have  very 
slightly  developed  hind  limbs,  reminding  us  of  the  fact  that 
they  are  descended  from  forms  which  had  well-developed 
limbs,  their  present  limbless  condition  being  secondary 
(Fig.  15).  Whales  also  have  vestiges  of  hind  limbs,  in  the 
form  of  certain  small  bones  lying  beneath  the  skin  and  not 
in  any  way  functional  (Fig.  16).  They  are  vestiges  of  the 


96 


ORGANIC  EVOLUTION 


functional  hind  limbs  possessed  by  the  terrestrial  ancestors 
of  the  whales.  Similarly,  the  Apteryx  of  New  Zealand, 
which  has  no  functional  wings,  has  vestiges  of  wings,  recall- 
ing the  typical  bird 
condition  (Plate  35). 
All  these  vestigial 
structures  are  with- 
out much  meaning 
until  we  recognize 
that  they  point  us  to 
the  ancestral  forms  in 
which  they  were  im- 
portant functional 
organs.  We  might 
give  many  illustra- 
tions of  such  vestigial  organs.  I  will  merely  mention  a  few 
found  in  man:  the  muscles  which  move  the  skin,  but  in 
most  persons  are  too  weakly  developed  to  do  so  except  in 


FIG.  15. —  Part  of  the  skeleton  of  a  boa  constrictor, 
showing  the  vestigial  bones  of  the  hind  limbs.  —  From  a 
specimen  in  the  United  States  National  Museum. 


FIG.  16.  —  Skeleton  of  Greenland  whale,  showing  the  vestigial  pelvic  bones  near  the  base  of 
the  tail.     [From  ROMANES,  after  FLOWER.] 

the  region  of  the  face;  the  muscles  that  should  move  the 
ears  but  usually  are  not  functional  (Fig.  1 7) ;  the  nictitating 
membrane,  vestigial  in  man,  but  well  developed  as  a  third 
eyelid  in  reptiles  and  birds  (Plate  36);  the  hair  of  the  body, 


PLATE  35.  —  Apteryx  australis. 

The  upper  figure  from  a  stuffed  specimen  in  the  Smithsonian  Institution  ;  the  lower  figure 
from  a  skeleton  in  the  museum  of  The  Woman's  College  of  Baltimore.  A  piece  of  black 
cardboard  has  been  placed  behind  the  skeleton  of  the  diminutive  wing. 


PLATE  36.  —  Eyes  of  various  vertebrates,  showing  the  nictitating  membrane,  indicated  by  the 
letter  N.  In  some  reptiles  and  birds  the  nictitating  membrane  can  be  drawn  over  the  whole  front 
of  the  eyeball.  —  From  Romanes'  Darwin  and  After  Darwin,  by  the  courtesy  of  The  Open  Court 
Publishing  Company. 


PLATE  37.  —  Hair  tracts  on  the  arms  and  hands  of  a  man  and  a  male  chimpanzee.  Drawn 
from  life.  Observe  that  in  the  corresponding  regions  the  direction  of  the  slope  of  the  hairs  is  the 
same.  —  From  Romanes'  Darwin  and  After  Darwin,  by  the  courtesy  of  The  Open  Court  Publish- 
ing Company. 


COMPARATIVE  ANATOMY 


97 


FIG.   17.  —  Muscles  of  the  human  ear.  — 
from  Gray's  Anatomy. 


reduced  to  a  mere  vestige  of 
what  we  see  in  the  apes,  the 
nearest  relatives  we  have  (Plate 

37). 

The  eyes  of  some  cave- 
dwelling  animals  are  among 
our  best  examples  of  vestigial 
structures.  In  Mammoth  Cave, 
for  example,  there  is  an  under- 
ground river  of  considerable 
size  in  which  are  found  fish 
and  Crustacea  whose  eyes  are 
in  different  stages  of  degenera- 
tion (Fig.  1 8).  Of  course,  liv- 
ing in  total  darkness  as  these 
animals  do,  they  can  have  no  use  for  eyes.  The  presence 
of  eyes  in  a  vestigial  condition  is  an  indication  of  the  fact 

that  these  cave-dwelling 
species  are  descended  from 
forms  which  once  lived  in 
the  outer  world.  As  eyes 
are  useless  to  animals  living 
in  the  dark,  natural  selection 
of  course  no  longer  will  keep 
the  eyes  perfect,  and  the 
degeneration  begun  by  the 
withdrawal  of  natural  selec- 

FIG.  18.  — Three  fishes,  showing  stages  in  the  ,.  -11  ,-ii       fnT.4-l,^r 

loss  of  eyrs  and  color.      A.  Dismal  Swamp  fish  "Oil       Will  gO       Still       further, 

(Chologaster  avefus),  thought  to  be  the  ancestor  i  •,  •  •,  •  j- 

of  the  blind  fish.     B.  Agassiz's  cave  fish  (Cholo-  because    it  IS    a    pOSltlVC    dlS- 
gaster agassizi) .      C.  Cave  blind  fish  (Typhlich-           -, 

tkys  sJter,-a,,eus).-  From  Jordan  and Kellogg's  advantage  to    any    SpCClCS    tO 

Animal  Life,  bv  the  courtesy  of  the  authors  and  ,  •  1 

of  D.Appieton&co.  waste   nutriment  on   useless 

H 


98  ORGANIC  EVOLUTION 

organs :  thus  in  time  the  eyes  will  become  mere  vestiges  of 
their  former  selves.  Weismann's  theory  of  germinal  selec- 
tion also  may  apply  here.1 

The  great  variety  of  forms  among  animals  and  plants, 
their  different  degrees  of  complexity,  the  phenomena  of 
homology  and  of  vestigial  structures,  are  readily  explained 
by  the  theory  of  evolution,  though  without  the  aid  of  this 
theory  they  are  apparently  meaningless  to  us. 


The  phenomena  of  embryology  as  related  to  the  theory  of 
evolution. 

In  the  study  of  the  anatomy  of  different  plants  and 
animals  we  find,  as  already  stated,  that  they  are  of  very 
different  degrees  of  complexity.  We  judge  in  general  that 
the  simpler  species  are  the  more  primitive  and  that  the  more 
elaborate  have  been  evolved  from  simpler  forms,  perhaps  from 
forms  more  or  less  like  some  we  find  living  to-day.  The 
study  of  embryology  gives  us  additional  evidence  of  the  truth 
of  this  conclusion.  We  find  that  complexly  organized  ani- 
mals and  plants  arise  each  from  a  single  cell,  the  fertilized 
egg,  and  gradually  acquire  new  organs  and  a  more  compli- 
cated structure,  till  finally  the  adult  condition  is  reached 
(Fig.  19).  The  series  of  stages  of  increasing  complexity, 
seen  in  the  development  of  one  of  these  higher  forms,  reminds 
us  of  the  taxonomic  series  in  our  classification  of  plants  and 
animals,  in  which  we  found  all  gradations  in  complexity  from 

1  For  a  brief  statement  of  the  essentials  of  the  theory  of  germinal  selection 
see  Appendix  I. 


EMBRYOLOGY 


99 


the  lowly  Protozoa  and  Protophyta  to   the    vertebrates  and 
flowering  plants. 

Not  only  do  we  find  that  there  are  these  two  kinds  of 
series,  the  anatomical  and  the  embryological,  but  we  find 
that  the  two  series  often  correspond  to  a  remarkable  degree. 
Take  an  illustration.  Among  the  vertebrates,  fishes  are  the 


FIG.  19.  —  Stages  in  the  development  of  the  pond  snail  (Lymnceus).     [After  HAECKKL.] 

simplest  on  the  whole.  The  Amphibia  are  in  general  some- 
what more  modified  in  their  organization.  The  reptiles  and 
birds  are  still  more  so,  and  the  mammals  are  in  some  re- 
gards the  most  highly  developed  of  all.  Now,  as  we  study 
the  embryology  of  the  Mammalia,  we  find  that  in  some 
features  of  their  general  organization  and  in  the  character  of 
many  of  their  separate  organs  the  different  stages  in  their 


100  ORGANIC  EVOLUTION 

development  correspond  to  the  conditions  seen  in  the  lower 
vertebrates  (Plate  38).  There  is  a  stage  when  the  mam- 
malian embryo  has  gill-slits  like  a  fish,  also  a  simple  tubular 
heart  and  a  blood  circulation  much  more  fish-like  than  is  the 
adult  mammalian  circulation.  This  we  interpret  as  a  remi- 
niscence of  the  time  when  the  ancestors  of  the  mammalia  were 
aquatic  animals.  Birds  and  reptiles  show  in  their  embry- 
ology a  similar  stage  resembling  the  fish  in  many  important 
regards.  The  frog  and  other  terrestrial  Amphibia  are 
actually  aquatic  in  early  life,  their  tadpoles  being  very 

fish-like  (Fig.  20). 

In  these  different  stages  in  the 

embryology  of  an  animal  we  read 

FIG.  20.—  Tadpole  of  salamander  (Am-       the     history    of     its    CVOlutlOn     from 

b/ystoma),  magnified  2j  times. 

simpler  forms  to  its  present  state. 

We  say  that  the  development  of  the  individual  tends  to 
recapitulate  the  evolution  of  the  race,  and  in  studying 
embryology  from  this  standpoint  we  are  studying  the 
racial  history. 

Many  examples  of  the  interpretation  of  race  histories 
from  the  study  of  embryology  might  be  given  among  both 
plants  and  animals.  I  will  give  but  one  more,  chosen  from 
the  higher  Crustacea.  The  Decapoda,  the  highest  group 
of  the  Crustacea,  includes  among  many  others  several  forms 
familiar  to  us  all:  the  lobster,  the  crawfish,  and  the  crab. 
The  lobster  (Plate  39)  has  the  posterior  part  of  the  body 
long  and  well  developed,  using  it  in  swimming,  and  by 
its  aid  the  lobster  is  able  to  leap  through  the  water  to  con- 
siderable distances.  We  call  this  portion  of  the  body  the 
abdomen.  It  is  filled  with  powerful  muscles,  and  is  divided 
into  seven  parts,  or  segments,  which  move  freely  upon  one 


PLATE  39.  —  Lobster  (Homarus  americanus)  t  two-filths  natural  size. 


PLATE  41.  —  A.  "  Mysis  stage"  in  the  development  of  the  American  lobster.  Each  leg  is  seen 
to  have  two  branches.  [After  HERRICK.]  B.  Mysis  stenolepis.  [From  GLAUS.]  C.  A  single 
leg  of  Mysis,  showing  its  two  branches.  [From  LANG.] 


EMBRYOLOGY  IOI 

another.  In  six  of  these  segments  are  ganglia  of  the  ner- 
vous system,  controlling  the  action  of  the  muscles  of  the 
several  segments  (Plate  40,  A).  The  crab  appears  to  be 
very  different  (Plate  40,  J3}.  There  does  not  at  first  sight 
seem  to  be  any  abdomen  at  all,  but  turn  the  crab  on  its 
back,  and  we  see  on  the  under  side  a  small  structure  cling- 
ing close  to  the  under  side  of  the  body,  which  when  care- 
fully examined  shows  the  same  divisions  into  segments  that 
we  observed  in  the  abdomen  of  the  lobster  (Plate  40,  B,  c]. 
It  is  the  abdomen  of  the  crab,  but  much  reduced  in  size,  and 
almost  functionless.  It  contains  no  nervous  ganglia  and  is 
very  different  apparently  from  the  abdomen  of  the  lobster. 
But  when  we  come  to  study  the  embryology  of  the  crab  we 
see  that  it  passes  through  a  stage  when  it  has  an  elongated 
abdomen  with  ganglia  in  six  of  its  seven  somites  (Fig.  21). 
This  lobster-like  stage  in  the  development  of  the  crab  is  a 
reminder  of  the  fact  that  the  crab  is  descended  from  ancestors 
resembling  the  lobster.  Let  us  go  a  little  farther.  The 
lobster  has  legs  like  those  of  a  crab,  consisting  of  a  linear 
series  of  joints.  In  the  embryology  of  the  lobster,  however, 
we  find  a  stage  when  the  legs  are  double,  not  single,  each 
leg  having  two  branches  (Plate  41,  A).  In  this  regard  the 
lobster  larva  resembles  another  member  of  the  group  Deca- 
poda,  namely  Mysis,  a  small  animal  with  which  many  may 
not  be  familiar  (Plate  41,  B  and  C).  We  call  the  stage  in 
the  development  of  the  lobster  when  its  legs  are  biramous  the 
Mysis  stage,  and  conclude  that  it  is  an  indication  that  the 
lobster  is  descended  from  Myszs-Yike  ancestors.  Some  crabs 
have  larvae  with  biramous  legs.  Of  course  conclusions  are 
not  drawn  from  a  single  indication  like  the  above,  but  the 
whole  condition  of  the  organism  is  studied.  For  the  sake  of 


IO2 


ORGANIC  EVOLUTION 


simplicity  we  have  noticed  in  each  case  but  a  single  feature 
of  the   comparison. 

The  embryological  repetition  of  the  race  history  is  gen- 
erally   much    distorted    by   secondary     modifications     which 


FIG.  21.  —  Three  stages  in  the  development  of  a  crab  (Cancer  pagurus}.     [After  HUXLEY.] 

A.  A  newly  hatched  larva.    B.  An  older  larva.     C,  D.  Much  older  larvae.     In  all  of  these  the 
elongated  abdomen  is  shown.    In  the  two  earlier  stages  some  of  the  legs  are  seen  to  be  biramous. 

cause  all  stages  in  the  life  history  to  become  more  perfectly 
fitted  for  their  life  conditions,  but  underneath  these  sec- 
ondary modifications  we  can  often  see  indications  of  the 


EMBRYOLOGY 


103 


character  of  the  ancestral  forms  to  which  the  several  em- 
bryonic stages  correspond. 

The  phenomena  of  homology  are  as  evident  in  the  study 
of  embryology  as  in  anatomy.  Many  structures  in  the 
embryo  can  be  properly  understood  only  after  comparison 
with  similar  organs  in  other  forms  to  which  they  are  related. 

Another  class  of  structures,  which  we  may  call  nascent 
organs,  appears  in  the  embryology  of  very  many  forms. 
These  are  organs  which 
begin  to  appear  during 
the  development  of  the 
animal  or  plant,  but 
which  never  become 
fully  developed  or  nor- 
mally functional,  and 
soon  disappear  before 
the  adult  condition  is 
reached.  They  recall 
some  ancestral  condition 
in  which  these  organs 
were  important,  and  are 

Of     interest      aS     Showing  B.  The  single  aperture  (mouth  anus)  by  which  the 

,  •     i      •<   .  i  digestive  cavity  opens  to  the  exterior. 

the    racial    history,    but, 

so  far  as  we  now  can  judge,  the  weakly  developed  rudiments 
of  these  structures  are  of  little  importance  to  their  present 
possessors.  Numerous  examples  might  be  given.  I  will 
mention  but  one. 

The  jellyfishes  and  their  relatives  have  but  a  single  open- 
ing into  their  alimentary  canal,  which  serves  both  for  the  inges- 
tion  of  food  and  the  egestion  of  wastes  (Fig.  22).  Most  of  the 


FIG.  22.  —  Hydra, 
section. 


A    diagrammatic   longitudinal 


104  ORGANIC  EVOLUTION 

higher  animals  when  adult  have  two  apertures  into  the  diges- 
tive tract,  the  mouth  and  the  anal  aperture,  but  in  their  devel- 
opment they  pass  through  a  stage  when  like  the  jellyfishes  they 
have  only  the^one  opening  (Figs.  23  and  24).  This  single  em- 
bryonic aperture  is  called  the  blastopore  and  is  a  reminiscence 
of  the  jellyfish  mouth.  In  certain  of  the  lower  vertebrates, 
the  frog  for  example,  we  find  the  blastopore  present  in  the 
embryo  and  well  formed  and  functional  (Plate  42,  A  and  B}. 


FIG.  23.  — Gastrula  of  a  coral  polyp  (Monaxenia  darwinii).     [After  HAECKEL.] 
a.  A  surface  view.    b.  A  longitudinal  section. 

Later  it  closes  and  disappears.  In  the  higher  vertebrates, 
on  the  other  hand,  the  blastopore  does  not  become  functional 
at  any  time  during  the  embryonic  life  (Plate  42,  C).  It  is 
a  nascent  organ.  It  begins  to  appear,  but  never  reaches 
normal  development,  and  later  disappears  without  ever  hav- 
ing come  to  its  typical  condition.  Its  presence  is  of  no  use 
to  its  possessor,  so  far  as  we  can  see,  but  the  fact  that  it  is 
there  in  a  rudimentary  condition  agrees  with  our  principle 
that  the  development  of  the  individual  tends  to  recapitulate 
the  evolution  of  the  race.  The  ancestors  of  the  vertebrates, 


PLATE  42.  —  A  section  of  a  gastrula  embryo  of  a  frog.  bp.  Blastopore.  —  From  Marshall's 
Vertebrate  Embryology,  by  the  courtesy  of  Smith,  Elder  and  Co.  B.  A  diagrammatic  longitudinal 
section  of  an  older  embryo  of  a  frog.  b.  Blastopore.  e.  A  layer  of  cells  which  will  become 
the  lining  of  the  alimentary  canal.  n.  A  rod  of  cells  (the  notochord)  which  later  is  replaced 
by  the  vertebral  column.  /.  The  so-called  primitive  streak  where  the  notochord  and  the  lining 
of  the  alimentary  canal  fuse  with  the  outer  layer  of  the  embryo,  forming  a  plug  of  cells  through 
which  opens  the  blastopore.  The  thickened  part  of  the  outer  layer  of  the  embryo,  on  the  upper 
side,  will  form  the  brain  and  spinal  cord.  C.  A  diagrammatic  longitudinal  section  of  the  upper 
portion  of  an  embryo  of  a  bird.  Reference  letters  as  in  Fig.  B,  with  which  this  figure  should  be 
compared.  The  blastopore  is  very  imperfectly  developed  and  does  not  open.  It  is  indicated  only 
by  a  thin  spot  in  the  primitive  streak,  which  soon  disappears. 


PALEONTOLOGY 


105 


we' believe,  had,  like  the  jellyfishes,  but  a  single  opening  into 
the  alimentary  canal.  The  lower  vertebrates  repeat  this  con- 
dition in  the  course  of  their 
embryonic  development.  The 
higher  vertebrates  no  longer 
use  the  blastopore  even  while 
embryos,  but  they  retain  it  as  a 
transient  rudiment.  Of  facts 
like  these  we  have  no  satis- 
factory explanation  except  the 
theory  of  evolution,  with  its 
corollary  that  the  development 
of  the  individual  tends  to  be 
a  recapitulation  of  the  race 
history. 

The  relation  of  the  phenom- 
ena of  paleontology  to  the  theory 
of  evolution. 

In  the  phenomena  of  com- 
parative anatomy  and  compara- 
tive embryology  we  see  much 
that  is  intelligible  only  with 


•  •,        r       i  f  FIG.  24.  — Longitudinal  sections  of  gas- 

thC    aid    Ot     the     theory    OI     CVO-       trulee of  various  animals.   [After  HAECKEL.] 

A.  Of  a.  worm,  Sag-/' tta.  B.  Of  a  starfish, 
Uraster.  C.  Of  a  crustacean.  D.  Of  a 
snail,  Lymncsus,  E.  Of  Amphioxus,  a  lowly 
relative  of  the  vertebrates.  d.  Digestive 

tual  record  of  this  evolution  in 


lution.      In   the  phenomena   of 

cavity,     o.  Blastopore. 

the  remains  of  the  animals  and  plants  which  have  lived  in 
the  past.  The  record  is  very  imperfect,  to  be  sure,  but  so 
far  as  it  goes  it  is  an  actual  record.  Only  very  unusual 


106  ORGANIC  EVOLUTION 

circumstances  will  secure  the  preservation  of  any  animal  or 
plant  as  a  fossil.  An  organism,  or  portion  of  an  organism, 
to  be  so  preserved  usually  must  be  hard  ;  it  must  be  buried 
beneath  soil  of  the  proper  kind,  and  when  buried  must  be 
impregnated  with  mineral  salts  or  in  some  other  way  pre- 
served from  disintegration.  When  once  converted  into  a 
fossil  it  must  escape  destruction  at  the  hands  of  those 
agencies  that  are  constantly  destroying  the  rocks,  heat,  press- 
ure, the  disintegration  that  comes  from  exposure  to  the 
atmosphere,  abrasion  by  ice,  and  especially  erosion  by  water. 
The  character  of  whole  continents  has  been  repeatedly 
changed  by  these  agencies.  No  wonder,- then,  since  fossiliza- 
tion  is  rare  and  the  destruction  of  fossils  when  once  formed 
so  easy,  that  our  record  of  past  faunas  and  floras  is  so  scant. 
It  is  a  cause  for  congratulation  that  we  have  so  much  of 
a  record  as  we  do  possess.  Thousands  of  species  of  fossil 
plants  and  animals  are  known,  and  as  yet  but  a  small  portion 
of  the  earth  has  been  searched.  We  will  give  attention  to 
but  a  few  illustrations  of  the  kind  of  record  we  find  in  the 
fossil-bearing  rocks,  choosing  naturally  records  that  are  quite 
complete. 

Let  us  first  look  at  a  table  showing  the  order  of  for- 
mation of  fossil-bearing  rocks.  At  the  bottom  of  the  table 
are  named  the  oldest  of  all  the  rocks  in  which  fossils  are 
known  to  be  found,  the  Cambrian  formation,  about  24,000 
feet,  four  and  one-half  miles,  in  thickness.  In  these  rocks 
we  find  fossil  remains  of  many  different  types,  jellyfish, 
sponges,  Polyzoa,  brachiopods,  echinoderms,  Molhisca,  and 
annulated  worms,  but  no  vertebrates.  Numerous  types  are 
represented,  but  they  were  simple  organisms  in  comparison 
with  the  representatives  of  the  same  types  found  in  the 


PALE  ONTOL  O  G  Y 


107 


PALEOZOIC  MKSO/.OIC  OENO/OIC 

Epochs  and  l-'ormations 

Faunal  Characters 

PLEISTOCENE. 
PLIOCENE,  3,00x3  ft. 
MIOCENE,  4,000  ft. 

OLKiOCENE,    8,000   ft. 

EOCENE,  10,000  ft. 

Man.     Mammalia  principally  of  living  species. 
Mollusca  exclusively  recent. 

Mammalia  principally  of  recent  genera  —  liv- 
ing species  rare.     Mollusca  very  modern. 

Mammalia  principally  of  living  families;    ex- 
tinct genera  numerous;    species  all  extinct. 
Mollusca  often  of  recent  species. 

Mammalia  with  numerous  extinct  families  and 
orders;   all  the  species  and  most  of  the  gen- 
era extinct.     Modern  type  shellfish. 

CRETACEOUS,  12,000  ft. 
Chalk. 

JURASSIC,  6,000  ft. 
Oolite. 
Lias. 

TRIAS,  5,000  ft. 
New  Red  Sandstone. 

Dinosaurian  reptiles;    pterodactyls  (flying  rep- 
tiles);  toothed  birds;    earliest  snake;    bony 
fishes;   crocodiles;   turtles;    ammonites. 

Earliest    birds;    giant    reptiles    (ichthyosaurs, 
dinosaurs,   pterodactyls);   ammonites;    clam 
an  1  snail  shells  very  abundant;   decline  of 
brachiopuds;   butterfly. 

First  mammalian  (marsupial)  ;  2-gilled  cephal- 
opods  (cuttle-fishes,   belemnites)  ;    reptilian 
footprints. 

PERMIAN,  5,000  ft. 

CARBONIFEROUS,  26,000  ft. 
Coal. 

DEVONIAN,  18,000  ft. 
Old  Red  Sandstone. 

SILURIAN,  33,000  ft. 
CAMBRIAN,  24,000  ft. 

Earliest  true  reptiles. 

Earliest  amphibian   (labyrinthodont)  ;    extinc- 
tion   of  trilobites;     first    crayfish;     beetles; 
cockroaches;    centipedes;   spiders. 

Cartilaginous  and  ganoid  fishes;    earliest  land 
(snail)     and     freshwater     shells;      shellfish 
abundant;    decline   of  trilobites;    May-flies; 
crab. 

Earliest    fish;    the    first    air-breathers    (insect, 
scorpion);    brachiopods  and  4-gilled  cephal- 
opods  very    abundant;     trilobites;     corals; 
graptolites. 

Sponges,  jellyfish,  annulated  worms,  Mollusca, 
brachiopods,  Polyzoa,  echinoderms  —  no  ver- 
tebrates. 

From  Romanes'  Darwin  and  after  Darwin,  slightly  modified. 


IOS  ORGANIC  EVOLUTION 

rocks  of  more  recent  formation.  Take  any  group  and  com- 
pare a  number  of  Cambrian  fossils  of  this  group  with  a  num- 
ber from  the  younger  rocks  and  we  find  the  younger  fossils 
decidedly  higher  in  their  organization.  In  the  rocks  formed 
during  the  Silurian  age,  which  succeeded  the  Cambrian 
period,  we  find  the  vertebrates,  fishes,  beginning  to  appear, 
and  the  earliest  air-breathing  animals,  insects  and  scorpions, 
also  animals  of  the  same  groups  that  we  found  represented 
in  the  Cambrian  rocks,  but  of  a  more  elaborate  structure. 
In  the  Devonian  period  cartilaginous  and  ganoid  fishes  and 
terrestrial  and  fresh-water  shells  are  among  the  most  inter- 
esting forms.  In  the  next  younger  rocks,  the  Carboniferous, 
appear  the  earliest  Amphibia  as  well  as  more  highly  organ- 
ized representatives  of  the  several  groups  of  invertebrates. 
The  earliest  reptiles  appear  in  the  Permian  rocks,  which 
follow  the  Carboniferous.  Mammals  and  birds  are  found 
in  the  rocks  of  the  succeeding  two  periods,  and  all  of  the 
groups  of  vertebrates  and  invertebrates  continue  to  be  repre- 
sented by  progressively  more  highly  differentiated  species, 
many  of  the  more  lowly  types  disappearing,  until  we  come 
to  the  present  age,  commonly  called  the  age  of  man.  This 
general  sequence  of  fossils,  the  simpler  giving  way  to  the 
more  complex  as  we  come  down  to  the  younger  rocks,  is 
a  most  impressive  thing,  and  is  one  of  the  chief  evidences 
that  evolution  has  taken  place. 

Turning  to  a  few  illustrations  of  the  origin  of  particu- 
lar species  or  organs,  we  find  the  same  principle  of  grad- 
ual increase  in  complexity  as  we  come  from  the  older  to 
the  younger  geological  formations.  Our  record  of  the  evo- 
lution of  branching  antlers  in  the  deer  is  fairly  complete 
(Fig.  25).  The  first  deer  in  the  early  Miocene  had  no 


Pl.ATE  43. — Antlers  of  a  stag,  showing  the  addition  of  new  branches  in  successive  years. — 
From  Romanes'  Darwin  and  After  Darwin,  by  the  courtesy  of  The  Open  Court  Publishing 
Company. 


PALEONTOLOGY  109 

antlers  at  all.  In  the  middle  Miocene  we  find  deer  with 
two-pronged  antlers  of  small  size  (Fig.  25,  A  and  B\  In 
the  upper  Miocene  and  lower  Pliocene  are  found  three- 
pronged  antlers  somewhat  larger  (Fig.  25,  C  and  D],  In 
the  later  Pliocene  we  meet  four-pronged  and  five-pronged 
antlers  and  still  larger  (Fig.  25,  E}.  In  the  Pleistocene 
clays  we  see  arborescent  antlers  like  those  of  the  modern 
deer  (Fig.  25,  F}.  It  is  especially  interesting  to  see  that 


A          B  c         D  E  F 

FIG.  25. —  Fossil  deer  antlers.     [From  ROMANES,  after  GAUDRY.] 

A  and  B.   Cervus  dicrocerus.      C.    C.  Matheronis.      D.    C.  paradinensis.     E.   C.  issiodorensis. 
F,    C.  sedgwickii. 

the  antlers  of  our  deer,  as  the  animal  grows  older,  pass 
successively  through  the  several  stages  we  find  in  the 
series  of  fossils  just  referred  to,  new  branches  being  added 
each  year  (Plate  43),  thus  again  illustrating  the  fact  that 
the  development  of  the  individual  tends  to  recapitulate 
the  history  of  the  evolution  of  the  race. 

In  Fig.  26  are  shown  drawings  of  seventeen  different 
varieties  of  fossil  Paludina  shells,  all  from  the  same  local- 
ity  in  Slavonia.  Paludina  is  a  fresh-water  snail,  and  indi- 
viduals similar  to  the  variety  figured  in  the  last  drawing 


10 


ORGANIC  EVOLUTION 


are  living  to-day  in  the  lakes  of  Slavonia.  These  lakes 
have  been  gradually  filled  up  by  the  silt  brought  into  them 
by  their  tributary  streams.  Careful  study  of  the  deposits 


FIG.  26.  —  Successive  forms   of  Paludina  from   the   tertiary  deposits   of  Slavonia.  —  From 
Romanes'  Darwin  and  after  Darwin,  by  the  courtesy  of  The  Open  Court  Publishing  Company. 

thus  formed  has  brought  to  light  a  remarkably  complete 
series  of  fossil  Paludina  shells.  The  uppermost  of  these, 
those  nearest  the  surface  and  last  deposited,  are  identical 
with  the  forms  now  living  in  the  same  region.  As  we  go 


PA  LE  ONTOL  OGY  III 

lower  we  find  shells  of  a  gradually  simpler  and  simpler 
form,  less  corrugated  and  with  less  irregular  aperture  and 
less  elongated  from  mouth  to  apex.  We  have  here  in  these 
fossils  a  most  complete  record  of  the  several  steps  in  the 
evolution  of  the  irregular,  rugose  shells  of  this  species  of 
pond  snail.  Such  a  series  points  almost  indisputably  to 
the  theory  of  descent  with  modification  for  its  explanation. 

There  are  many  indications  of  close  resemblance  between 
birds  and  reptiles,  but  the  descent  of  the  former  from  the 
latter  is  most  clearly  shown  by  the  numerous  fossil  forms 
which  bridge  the  gap  between  the  two  groups.  Notice  the 
accompanying  drawings  of  three  of  these  intermediate  forms: 
Archaopteryx  (Plate  44);  Hesperomis  (Plate  45,  A)\  and 
Ichthyornis  (Plate  45,  B).  Compare  these  drawings  with 
Plate  45,  C,  which  represents  the  skeleton  of  one  of  the 
ancient  flying  reptiles,  and  with  the  skeleton  of  a  modern 
bird  as  shown  in  Fig.  27.  The  intermediate  forms  first  fig- 
ured so  approach  the  character  of  the  flying  reptiles  as  to 
strongly  indicate  that  they  are  descended  from  the  latter, 
but  they  are  true  birds.  The  fact  of  the  development  of 
the  birds  from  the  reptiles  is  very  clearly  indicated  in  the 
discovered  fossils  which  are  intermediate  in  structure  be- 
tween the  two  types. 

One  further  illustration  will  be  sufficient.  The  record 
of  the  origin  of  the  horse,  worked  out  by  American  paleon- 
tologists from  American  fossils,  is  probably  the  best  example 
of  paleontological  evidence  of  evolution.  The  horse  is 
especially  peculiar  in  the  character  of  its  feet  and  teeth,  and 
we  will  direct  our  attention  to  these  points  as  shown  in  the 
accompanying  illustrations.  In  the  lower  Eocene  rocks 
we  find  an  animal,  Phenacodus,  about  the  size  of  a  fox, 


I  12 


ORGANIC  EVOLUTION 


having  five  well-developed  toes  on  each  foot,  and  with  short 
and  but  moderately  corrugated  teeth  (Plate  46).  This  is  one 
of  the  simplest  known  relatives  of  the  hoofed  mammals ;  and 


FIG.  27. —  Skeleton  of  a  crow  (Corvus  americatta.}.  Observe  that  there  are  no  teeth  in  the 
jaws,  that  the  fingers  are  reduced  in  number  and  partially  fused  together,  and  that  the  skeleton 
of  the  tail  is  short,  ending  in  an  enlarged  bone  (to  which  the  chief  tail  feathers  are  attached). 

from  forms  something  like  Phenacodus  must  have  been 
developed  the  elephant,  rhinoceros,  hog,  sheep,  camel,  and 
all  the  other  hoofed  mammals,  including  the  horse  and  its 
long  line  of  ancestors.  Observe  the  steps  in  the  transfer- 


I  g 

w  -° 
£.i 

II 


Equns :  Qua- 
ternary and 
Recent. 


Miohipfius : 
Pliocene. 


Protohippus  : 
Lower  Plio- 


Orohippus : 
Eocene. 


PLATE  47. —  Diagrams  illustrating  gradual  changes  in  foot  structure  and  pattern  of  ridges  on 
the  crowns  of  the  molar  teeth  in  fossil  and  recent  species  of  the  horse  family.  [After  MARSH.] 

a.  Bones  of  the  fore  foot.  b.  Bones  of  the  hind  foot.  c.  Bones  of  the  fore  arm  (radius  and 
ulna),  d.  Bones  of  the  lower  leg  (tibia  and  fibula) .  e.  side  view  of  molar  tooth,  f,  g.  grinding 
^surfaces  of  upper  and  lower  molar  teeth,  showing  the  grinding  ridges. 


GEOGRAPHICAL   DISTRIBUTION  113 

mation  of  the  five-toed  limb  of  a  form  like  Phenacodus  into 
the  one-toed  limb  of  the  horse  (Plate  47).  Notice  also  the 
increasing  complexity  of  the  ridges  on  the  grinding  surface 
of  the  teeth  of  the  same  species  from  which  the  illustrations 
of  foot  structure  are  taken.  We  have  here  a  very  complete 
paleontological  record  of  a  profound  change  of  structure, 
giving  us  the  actual  history  of  the  evolution  of  the  horse. 

Geographical  distribution. 

The  comparison  of  the  phenomena  of  paleontology, 
anatomy,  and  embryology  seems  to  point  us  very  clearly 
to  the  theory  of  evolution  as  the  solution  of  the  problem 
of  origin.  It  is  interesting  also  to  find  that  the  distribution 
of  animals  and  plants  over  the  earth  is  such  as  this  theory 
would  lead  us  to  expect.  We  find  the  character  of  the 
fauna  and  flora  decidedly  different  in  different  regions  of 
the  earth,  and  these  differences  are  not  due  solely  to  differ- 
ences of  climate  and  soil  and  other  conditions  of  the  envi- 
ronment. Similar  environmental  conditions  do  not  produce 
similar  animals  and  plants  if  the  regions  compared  be  sepa- 
rated from  each  other  by  sufficient  distances  or  by  barriers 
that  prevent  free  migration  and  interbreeding.  The  phe- 
nomena of  distribution,  as  we  find  them,  agree  with  the 
hypothesis  that  the  different  species  of  animals  and  plants 
have  each  arisen  at  some  particular  place  and  have  spread 
from  that  spot,  becoming  modified  to  a  greater  or  less 
extent  during  their  wandering. 

In  general,  we  may  say  that  the  degree  of  intimacy  in 
relationship  between  the  faunas  and  floras  of  any  two 
regions  is  in  inverse  ratio  to  the  degree  to  which  barriers 


1 1 4  OR  GANIC  E  VOL  UTION 

are  present  between  these  two  areas  to  prevent  free  pas- 
sage from  one  to  the  other.  There  is  also  a  correlation 
between  the  kinds  of  barriers  present  and  the  kinds  of 
animals  and  plants  held  in  check  by  them.  Aquatic  ani- 
mals and  plants  are  restricted  by  the  intervention  of  land 
areas.  Terrestrial  organisms  are  held  back  by  the  presence 
of  large  bodies  of  water.  Animals  and  plants  adapted  to 
warm  climates  may  be  unable  to  cross  high  mountain  ranges 
whose  summits  will  have  a  cold  climate.  Dry  regions  will 
check  organisms  which  are  adapted  to  life  in  fertile  areas. 
Desert  species  will  not  readily  pass  a  forest  barrier  or  a 
region  of  marshes. 

Observe  the  conditions  on  some  of  the  islands  off  the 
west  coast  of  South  America.  Their  faunas  and  floras, 
while  different  from  those  of  the  mainland  because  of  their 
isolation  and  different  environment,  are  still  quite  closely 
related  to  those  of  the  mainland,  presenting  just  the  con- 
ditions we  would  expect  on  the  supposition  that  they  are 
descended  from  forms  which  migrated  from  the  mainland  at 
some  remote  period,  migration  having  since  been  suspended. 
Similarly  we  explain  the  resemblance  between  the  fauna 
and  flora  of  the  west  coast  of  North  America  and  those  of 
eastern  Asia  by  the  fact  that  at  one  time,  when  the  climate 
of  Alaska  was  mild,  migration  across  Behring  Straits  was 
possible,  and  by  our  belief  that  the  Asiatic  forms  once  estab- 
lished in  this  country  and  American  forms  once  having 
crossed  into  Asia,  communication  having  then  been  broken 
off,  the  forms  thus  separated  would  diverge  by  evolution. 

The  flora  of  the  higher  altitudes  in  the  White  Mountains 
of  New  Hampshire  shows  a  remarkable  resemblance  to  that 
of  Labrador.  This  suggests  that  the  White  Mountain  flora 


GEOGRAPHICAL  DISTRIBUTION  115 

is  a  remnant  of  the  arctic  flora  which  was  spread  over  New 
England  during  the  later  glacial  period,  and  that,  as  the  ice 
melted  and  the  arctic  flora  retreated  northward,  some  species 
persisted  in  more  southern  latitudes  by  ascending  the  moun- 
tains, the  cold  of  whose  higher  altitudes  resembles  the  arctic 
climate  to  which  these  species  are  adapted. 

Certain  cases  of  distribution  which  at  first  glance  seem  to 
be  anomalous  are  found  on  careful  scrutiny  to  support  our 
hypothesis.  For  example,  the  opossums  of  North  and  South 
America  are  very  different  from  all  the  other  mammals  of  the 
same  region,  so  different  as  to  be  properly  placed  in  a  distinct 
subclass,  the  Marsupialia.  In  no  other  region  are  similar 
animals  found  except  in  Australia  and  its  adjacent  islands.  In 
Australasia,  however,  there  are,  with  two  exceptions,  no  indig- 
enous mammals  except  those  belonging  to  the  same  subclass 
as  the  opossum.  It  seems  at  first  sight  absurd  to  postulate  any 
communication  between  Australasia  and  America  by  which 
one  may  have  become  peopled  from  the  other.  It  looks  as  if 
the  opossum  type  must  have  arisen  independently  in  the  two 
areas,  a  thing  which  would  be  contrary  to  our  knowledge  of 
the  ways  of  evolution.  Paleontology  here  comes  to  our  aid. 
The  fossil  fauna  of  America  is  rich  in  species  of  the  opossum 
type,  the  opossums  being  the  only  living  representatives  of  an 
at  one  time  very  extensive  marsupial  fauna.  The  marsupial 
type  is  more  primitive  than  that  of  the  other  Mammalia. 
There  is  evidence  that  at  one  time,  before  the  higher  Mamma- 
lia came  into  existence,  the  marsupials  were  spread  over  the 
whole  eastern  and  western  hemispheres,  and  that  as  the  higher 
mammals  arose  they  exterminated  the  mammals  of  the  more 
primitive  marsupial  type,  except  that  in  Australia  the  earlier 
forms  persisted  and  in  America  the  opossums  remained. 


Il6  ORGANIC  EVOLUTION 

Why  the  opossums  were  preserved  in  spite  of  the  compe- 
tition of  the  more  perfect  higher  Mammalia  we  cannot  say, 
but  we  do  know  probably  how  the  marsupials  of  Australia 
managed  to  persist.  There  is  reason  to  believe  that  the  con- 
tinent of  Australia,  or  the  chain  of  islands  to  the  north  of  it, 
was  once  connected  with  the  Malay  Peninsula,  so  that  the 
mammals  of  that  time,  which  we  believe  were  marsupials, 
could  readily  pass  from  one  region  to  the  other.  At  this 
time  apparently  much  of  the  earth  was  peopled  by  the  Mar- 
supialia.  When,  however,  Australasia  was  separated  from 
southeastern  Asia  by  the  formation  of  the  deep  straits  south- 
east of  Sumatra  (Fig.  28),  communication  between  the  two 
continents  was  cut  off  and  the  marsupials  of  Australasia 
were  thus  protected  from  competition  with  the  higher  mam- 
mals which  soon  arose  upon  the  larger  continent.  The 
mammals  of  the  higher  type  spread  over  Asia,  Europe, 
Africa,  and  North  and  South  America,  and  replaced  the 
marsupial  forms.  The  peculiar  distribution  of  the  Mar- 
supialia,  therefore,  instead  of  arguing  for  the  independent 
origin  of  the  marsupials  in  two  regions,  is  a  beautiful  exam- 
ple of  the  support  given  to  the  theory  of  evolution  by  the 
phenomena  of  geographical  distribution  when  studied  in  con- 
nection with  the  phenomena  of  paleontology,  geology,  and 
comparative  anatomy.  Other  striking  examples  might  be 
quoted,  but  this  will  suffice  to  show  the  general  relation  of 
these  phenomena  of  distribution  to  the  theory  of  evolution. 

The  fact  that  great  weight  is  given  by  students  of  zoology 
and  palaeontology  to  the  phenomena  of  geographical  distri- 
bution is  evidenced  by  a  belief  which  is  becoming  more 
general  among  paleontologists  ;  namely,  that  there  was  at 
one  time  a  great  Antarctic  continent  connecting  South  Africa, 


GE O GRAPHICAL   DISTR1B  UTION 


117 


South  America,  New  -Zealand,  and  perhaps  Australia.     This 
belief  is  based  upon  the  close  resemblance  in  many  remark- 


FlG.  28. —  Map  of  southeastern  Asia,  the  East  Indies,  and  Australia.  The  heavy  black  line 
southeast  of  Bali,  Borneo,  and  the  Philippine  Islands  indicates  the  deep-water  straits  which  sepa- 
rate the  Asiatic  fauna  from  the  Australasian  fauna.  The  sharp  contrast  between  the  terrestrial 
faunas  in  these  two  regions  makes  it  probable  that  this  line  of  demarcation  is  an  ancient  one. 

able    particulars    between   the  fossil  faunas  of   these  several 
southern    regions,    no    connecting   links    between  which  are 


Il8  ORGANIC  EVOLUTION 

found  among  the  fossils  of  the  northern  hemisphere.  It 
would  at  first  thought  seem  preposterous  to  postulate  the 
former  presence  of  such  a  connecting  continent  with  no  more 
evidence  in  its  favor  than  the  resemblance  between  these 
fossil  faunas.  Yet  this  line  of  evidence  has  proven  so  trust- 
worthy in  other  instances  that  some  of  our  most  conservative 
paleontologists  are  inclined  to  accept  the  evidence  in  this 
case  and  to  believe  that  such  a  continent  once  existed. 


Color  in  animals. 

The  phenomena  of  color  in  both  animals  and  plants  are 
among  the  most  remarkable  and  interesting  in  the  whole 
realm  of  nature.  It  is  not  so  much  the  way  in  which  the 
color  is  produced,  whether  by  pigments  or  by  refraction,  that 
interests  us  in  this  connection,  as  it  is  the  uses  to  which  the 
colors  are  put.  Let  us  first  refer  to  the  colors  of  animals. 

According  to  the  uses  to  which  colors  of  animals  are  put, 
we  may  classify  them,  for  purposes  of  description,  as  follows: '  — 

Indifferent  coloration,  not  useful,  so  far  as  we  can  judge ; 

Colors  of  direct  physiological  value ; 

Protective  coloration  and  resemblances ; 

Aggressive  coloration  and  resemblances ; 

Alluring  coloration  and  resemblances  ; 

Warning  coloration ; 

Immunity  coloration ; 

Mimetic  coloration  and  resemblances ; 

A,  Protective, 

B,  Aggressive ; 

1  In  the  main  I  have  followed  the  classification  used  in  Poulton's  delightful  book 
The  Colours  of  Animals. 


COLOR  IN  ANIMALS  119 

Signals  and  recognition  marks; 

Confusing  coloration  ; 

Sexual  coloration. 

We  are  not  interested,  in  this  connection,  in  non- 
useful  colors,  or  in  the  direct  physiological  value  of  colors. 
The  other  uses  of  color,  however,  present  a  diverse  series 
of  phenomena  very  significant  in  the  light  of  the  theory 
of  evolution. 

Protective  coloration  and  resemblances. 

We  referred  in  the  early  pages  of  this  book  to  the  severity 
of  the  struggle  for  existence  and  to  the  importance  of  any 
structure  or  character  which  enables  its  possessor  to  escape 
destruction.  Carnivorous  animals  are  so  common  and  so 
voracious  that,  as  we  would  naturally  expect  to  find,  their 
prey  have  adopted  various  means  of  defence.  Among  these 
some  of  the  most  important  have  to  do  with  color.  Ani- 
mals which  closely  resemble  their  environment  in  color 
will  escape  the  notice  of  their  enemies  and  thus  be  pre- 
served, while  their  less  protectively  colored  neighbors  will 
be  seen,  captured,  and  devoured.  Natural  selection  will 
thus  tend  to  produce  protective  coloration.  The  principle 
must  be  sufficiently  clear.  Let  us  observe  a  number  of 
instances  of  such  coloration. 

Many  animals  which  live  at  the  surface  of  the  open 
ocean  are  transparent,  so  as  to  be  distinguished  only  with 
difficulty  from  the  water  itself.  This  is  true  of  many  of 
the  jellyfishes  and  their  relatives  the  ctenophores  and 
siphonophores,  of  most  pelagic  Crustacea  and  worms,  of 
the  pelagic  tunicates,  and  many  other  less  familiar  forms, 
and  of  almost  all  marine  larvae.  This  invisibility  must  be 


120  ORGANIC  EVOLUTION 

a  most  effective  means  of  protection  to  these  transparent 
forms. 

Fish  are  commonly  dark-colored  above  and  light-colored 
below.  To  any  enemy,  such  as  a  sea-gull,  looking  down 
upon  them  from  above,  their  dark  color  would  cause  them 
to  harmonize  with  the  dark  appearance  of  the  water,  while 
another  fish  looking  at  them  from  below  or  from  the  side 
would  be  less  likely  to  detect  them  than  if  they  were 
dark-colored  instead  of  light-colored  beneath.  Were  the 
lower  surface  as  dark-colored  as  the  dorsal  surface  it  would 
appear  to  be  much  darker  still,  because  of  its  being  in  shadow. 
The  light-colored  sides  and  belly  of  most  fish,  when  the 
light  comes  upon  the  fish  from  above,  are  shaded,  and 
being  in  shadow  appear  about  as  dark  as  the  dorsal  sur- 
face. If  the  sides  and  ventral  surface  were  actually  dark- 
colored  the  added  shadow  would  make  them  seem  very 
dark  and  would  make  the  fish  conspicuous.  The  accom- 
panying photograph  of  a  bluefish,  taken  while  the  fish 
was  swimming  in  an  aquarium  with  the  light  coming  from 
above,  shows  the  really  brilliant  white  sides  and  belly  ap- 
parently as  dark  as  the  steel-blue  back,  because  of  their 
being  in  shadow  (Plate  48,  A).  The  color  of  most  fish 
resembles  that  of  their  environment.  The  flatfish  and  others 
which  live  upon  or  near  the  bottom  often  closely  resemble 
the  bottom  in  color  (Plate  48,  B}. 

Most  birds  are  so  colored  as  to  conform  to  the  sur- 
roundings in  which  they  live.  Think  for  a  moment  of  the 
sparrows,  streaked  and  speckled  browns  and  grayish  browns 
like  the  grasses  and  bushes  among  which  they  are  com- 
monly found  (Plate  49,  A)\  of  the  whole  grouse  tribe,  the 
quail  (Plate  49,  ^),  the  pheasants,  the  ruffed  grouse  (Plate 


PLATE  48.  —  A.  Bluefish  (Pomatomus  salfatrix).  —  From  a  photograph  from  life  by  A.  R.  Dugmore, 
published  in  Jordan  and  Evermann's  American  Food  and  Game  Fishes.  By  permission  of  Doubleday, 
P.'ge  and  Co.  B.  Photograph  of  a  living  flat-fish,  "sand  flounder"  (Paralicfithys  dentata) .  It  is  lying 
upon  clean  white  sand.  Against  an  ordinary  sand  bottom  its  mingled  grays,  browns,  and  greens  would 
render  it  almost  indistinguishable.  It  is  interesting  to  observe  that  the  circular  markings  with  dark 
centres  closely  resemble  shadows  of  bubbles.  The  much  darker  "  mud-flounders "  are  almost  equally 
well  protected  by  their  resemblance  in  color  to  the  dark  mud  against  which  they  lie. 


II 

lo 

-   ~ 


I! 
II 


PLATE  51. — A.  A  nighthawk  (dead)  upon  an  oak  log.     B.  A  humming-bird's  nest  upon  a  pine 
branch.  —  From  an  exhibit  in  the  United  States  National  Museum. 


COLOR  IN  ANIMALS  121 

23),  and  the  jungle  fowl  from  which  our  domestic  fowl 
are  descended  (Plate  16,  A\  all  of  which  are  colored  more 
or  less  like  the  sparrows  and  have  a  similar  habitat.  Think 
of  the  snipe  tribe,  including  the  shore  birds  like  the  sand- 
pipers, the  curlew,  and  the  woodcock.  The  woodcock  in 
its  native  haunts  is  almost  invisible  (Plate  50).  I  have 
shot  scores  of  them,  yet  have  never  but  once  seen  one  of 
them  upon  the  ground,  and  this  too  in  spite  of  the  fact 
that  I  have  had  a  dog  with  me  on  all  of  my  shooting  trips, 
and  he  would  stand  pointing  the  bird,  often  for  a  long  time 
before  the  bird  would  rise. 

The  bright  green  color  of  some  tropical  birds,  like  cer- 
tain of  the  parrots,  is  to  them  a  most  effective  protection. 
In  Jamaica  there  is  a  small  bright  green  bird,  the  "green 
tody."  While  spending  a  summer  in  zoological  study  in 
Jamaica  I  wanted  to  shoot  one  and  bring  home  its  skin 
to  show  as  an  illustration  of  protective  color.  Often  when 
out  with  my  gun  I  heard  the  faint  piping  whistle  of  one 
of  these  little  fellows  and  searched  carefully  for  him,  but 
always  without  success.  They  rarely  fly  when  one  is  near 
them,  seeming  instinctively  to  rely  for  protection  upon  their 
color  while  they  remain  motionless  among  the  green  leaves. 
Once  I  thought  I  was  at  last  to  be  successful,  for  I  located 
a  tody  in  a  drooping  branch  of  a  tree  where  I  could  walk 
all  around  him  and  thoroughly  inspect  the  whole  branch. 
Yet,  though  I  came  within  six  feet  of  the  branch,  peering 
among  the  leaves  in  every  part,  I  could  not  recognize  the 
bird.  Finally  I  drew  away  about  five  rods  and  fired  into 
the  branch,  but  the  bird  escaped,  for  I  fired  too  high.  He 
had  been  within  six  feet  of  my  eyes  during  the  whole  of 
my  closest  search.  (See  also  Plate  51.) 


122 


ORGANIC  EVOLUTION 


Most  snakes,  lizards,  and  frogs  are  protectively  colored. 
Our  common  eastern  tree-lizard,  which  is  found  often  on  the 
gray,  lichen-covered  bark  of  the  scrub  pines,  is  a  mottled 
greenish  gray  and  is  hardly  distinguishable  from  the  bark 
(Plate  52).  Most  snakes,  living  as  they  do  upon  the  ground, 
are  dull  colored,  gray  or  brown,  or  dull 
blackish,  like  the  shadows  among  the 
bases  of  the  grass  stalks.  One  beauti- 
ful little  snake,  found  throughout  the 
eastern  United  States,  is  a  bright  green, 
and  at  first  thought  it  seems  very  con- 
spicuously colored,  but  it  is  a  climber, 
living  a  large  share  of  the  time  in  the 
branches  of  low  shrubs,  where  its  color 
renders  it  inconspicuous  among  the 
green  leaves.  It  is  interesting  to  note 
that  when  disturbed  this  snake  is  very 
likely  to  seek  safety  by  flight  into  the 
bushes  rather  than  along  the  ground. 
Deer,  rabbits,  antelope,  wild  sheep, 
and  goats,  and  most  other  mammals,  are 
dull-colored  and  resemble  the  region  in 
which  they  live  (Plates  53  and  54,  A). 

Most  insects  show  protective  colora- 
tion (Plates  55  and  56),  and  so  do  crabs,  lobsters,  crawfish, 
and  most  other  Crustacea.  This  is  true  also  of  the  spiders, 
most  of  which  are  inconspicuously  colored.  Most  species 
are  dull  brown  or  gray,  like  the  dead  leaves,  bark,  or  lichens 
upon  which  they  are  found  (Fig.  29);  some  are  green,  like 
living  foliage  (Plate  85,  D\  The  members  of  one  family, 
which  live  usually  within  the  blossoms  of  flowers,  are 


FIG.  29.  —  A  straw-colored 
spider  (  Tetragnathagrallator) 
in  its  accustomed  position  on 
a  blade  of  dead  grass.  —  From 
a  specimen  given  by  H.  W. 
Britcher. 


PLATE  52.  —  TREE  LIZARDS  (Sceloporus  undulatus)  ON  OAK  BARK. 


PLATE  54.  —  A.  A  "cony"  or  "pika'1  ( Otochona  princeps)  among  rocks. — From  a  photo- 
graph by  E.  R.  Warren.  B.  A  protectively  colored  woods-moth  {Homoptera  edusa)  on  a  piece 
of  bark. 


<         f        \X          <        ' 


PLATE  55.  —  PROTECTIVELY  COLORED  WOODS-MOTHS.    [After  PACKARD  and  KAPPEL  AND 

KIRBY.] 

A.  Sphinx  convolvuli.  B.  Leucania  l-album.  C.  Phorodesmiu  smargdaria.  D.  Smerinthus 
tilicB.  E.  Dasychira  pudibunda  9.  F.  Eriopus  purpureofasciata.  G.  Diatithcecia  compta. 
H.  Panthia  ccenobitu.  1.  Ichthyura  inclusa,  var.  inve/sa.  J.  Cidaria  °aliata.  K.  Cidaria  ocel- 
lata.  L.  Aplecta  occultii.  At.  Hetei  ocampa  pulverea. 


PLATE  58. — Grass  porgy  (Calamus  arctifrons) ,  showing  changes  in  color  occurring  in  a  few 
moments. —  From  photographs  by  A.  R.  Dugmore  in  Jordan  and  Evermann's  American  Food  and 
Game  Fishes,  by  permission  of  Doubleday,  Page  and  Co. 


PLATE  59.  —  Color  adaptation  in  pupae  of  Pieris  rapce  and  Vanessa  urticce.     [After  POULTON.] 

a.  \magoofPierisrapce.  b-k.  Pupae  of  Pieris  rapce.  l-q.  Pupae  of  Vanessa  urticce.  r.  Imago 
of  Vanessa  urticce. 

The  color  of  these  pupas  has  been  determined  by  placing  the  caterpillars,  when  nearly  ready  to 
pupate,  in  boxes  lined  with  different  colored  papers  (black,  red,  yellow,  green).  Each  pupa  con- 
forms more  or  less  closely  to  the  color  of  the  lining  of  the  box  in  which  it  was  formed.  • 


COLOR  IN  ANIMALS  123 

brightly  colored  like  the  blossoms,  their  color  rendering 
them  inconspicuous  (Plate  75,  A).  Spiders  are  exposed  to 
the  attacks  of  enemies,  especially  of  certain  wasps  which 
capture  them,  paralyze  them  by  stinging  them,  and  then 
use  them  to  provision  their,  nests,  the  young  wasps  feeding 
upon  the  living  spiders.  They  therefore  need  protection. 

Of  special  interest  are  the  protective  seasonal  changes 
of  color,  seen  in  some  northern  animals;  for  example,  sev- 
eral species  of  ptarmigan  and  the  New  England  and  Cana- 
dian hare,  which  are  white  in  winter,  resembling  the  snow, 
are  grayish  or  brownish  in  summer  like  the  dead  leaves  and 
the  rocks  among  which  they  are  found,  while  in  the  spring 
and  fall,  while  shedding  their  feathers  or  hair,  they  are  a 
spotted  gray  and  white  or  brown  and  white,  bringing  them 
into  color  harmony  with  their  environment,  in  which  patches 
of  snow  are  scattered  among  the  rocks  or  leaves  (Plate  57). 

Some  animals  are  able  rapidly  to  change  their  color, 
thus  keeping  them  in  harmony  with  the  varying  color  of 
their  surroundings  as  they  move  from  place  to  place.  The 
chameleon,  the  little  Anolis  of  our  southern  states,  some 
frogs,  and  many  kinds  of  fishes,  especially  tropical  fishes, 
have  this  ability  (Plate  58). 

It  is  well  known  that  the  pupae  of  most  butterflies 
are  colored  to  correspond  to  their  environment.  Professor 
Poulton,  experimenting  upon  certain  species  of  butterflies, 
has  shown  that  by  placing  the  full-grown  caterpillars  in 
boxes  lined  with  different  colored  paper,  pupae  of  colors 
corresponding  to  that  of  the  paper  with  which  they  were 
surrounded  can  be  obtained  (Plate  59). 

There  are  many  instances  of  special  resemblance,  in 
color  or  in  form  or  in  both,  between  a  species  of  animal 


K 


I24  ORGANIC  EVOLUTION 

and  some  particular  object,  the  animal  escaping  detection 
because  of  this  resemblance.  Often  the  animal  has  peculiar 
habits  which  make  the  resemblance  more  perfect.  Among 
insects  these  special  resemblances  are  not  uncommon.  One 
of  the  best  examples  is  the  caterpillar  of  the  brimstone  moth, 
which  resembles  a  twig,  and  which  remains  motionless  in 
just  the  position  to  make  this  resemblance  most  perfect.  In 

color,  shape,  and  habit- 
ual position  the  resem- 
blance is  very  exact. 
The  caterpillars  of  many 
other  species  of  moths 
show  a  similar  resem- 
blance to  twigs  (Fig.  30 
and  Plate  60).  Some 
caterpillars  resemble 
the  ragged  edges  of  the 
leaves  of  their  food- 
plant,  both  color  and 
shape  making  a  striking 

FIG.  30.  —  Twig-like  caterpillar  of  the  moth  Selenia        rCSCmblanCC     (Plate      56). 
tetralunaria,  on  a  spray  of  birch.     [After  WEISMANN.] 

A'.  The  head.      F.  The  feet.     M.  A  mark  resem-        Other       Caterpillars       are 

green  with  brown  spots, 

conforming  closely  in  color  and  color  pattern  to  the  fungus- 
spotted  leaves  upon  which  they  are  found  (Plate  56).  Some 
.adult  insects  resemble  sticks ;  for  example,  the  common 
"walking-stick"  (Plate  61,  A\  In  Nicaragua  there  is 
found  a  walking-stick  in  which  the  deception  is  carried  still 
farther  by  certain  excrescences  on  the  body  and  legs  which 
cause  it  to  resemble  a  bit  of  moss  (Plate  61,  B\  Belt,  its 
discoverer,  says  it  is  found  on  moss.  Many  insects  resemble 


PLATE  60.  — Caterpillar  of  the  moth  Catocala  amatrix,  on  a  poplar  twig. 

A.  Indicates  its  head.  B.  Its  posterior  end.  The  bark  of  the  young  twigs  of  this  tree  is  of  a 
peculiar  purplish  gray  color.  The  caterpillar  not  only  imitates  this  color  to  perfection,  but  it  also 
has  the  habit  of  so  flattening  itself  against  the  twig  as  to  appear  a  part  of  the  twig  itself.  This 
caterpillar  on  a  leafy  spray,  while  alive,  was  handed  at  different  times  to  four  biologists  with  the 
remark,  "  Isn't  that  a  fine  example  of  protection  ?  "  and  none  of  them  saw  the  caterpillar. 


PLATE  61.  —  A.  Three  "walking-sticks"  on  a  twig.  The  two  larger  ones  are  of  the  species 
Diapheromera  femorata.  —  From  an  exhibit  in  the  United  States  National  Museum.  B.  An 
insect  which  lives  upon  moss  and  which  closely  resembles  the  moss  in  form  and  color  (green). 
[After  BELT.] 


PLATE  62.  —  A.  A  green  locust  which  resembles  a  leaf.  It  is  probably  a  species  of  Cycloptera. 
[After  BEDDARD.]  B.  A  leaf-like  mantis  (PhylHum  siccifolium).—  Vrom.  Brehm's  Thierleben. 
C.  A  longicorn  beetle  (Mormolyce  phyllodes) .  —  From  Brehm's  Thierleben. 


PLATE  63.  —  Logoa  opercularis  and  Logoa  crispata.     About  natural  size. 

A.  Cocoon  of  L.  opercularis.  B.  Larva  of  L.  opercularis.  C.  Dorsal  view  of  larva  of  L. 
crispata.  D.  Side  view  of  larva  of  L.  crispata.  E.  Cocoon  of  L.  crispata,  with  moth  emerging. 
F.  Imagines  of  L.  opercularis :  upper  figure,  male ;  lower  figure,  female.  B,  C,  Dt  and  E  drawn 
from  specimens  lent  by  the  United  States  National  Museum. 


COLOR   IN  ANIMALS  125 

leaves.  We  have  leaf-like  grasshoppers,  leaf-like  Mantides, 
leaf  beetles  (Plate  62),  leaf  moths,  and  leaf  butterflies  (Plate 
83,  B,  D,  E,  K}.  There  are  a  number  of  the  latter  in  this 
country,  but  the  finest  example  is  Kallima  inackis,  found  in 
India.  In  this  species  the  resemblance  to  a  dead  leaf  is  almost 
perfect  when  the  wings  are  closed  (Plate  83,  A  and  B\ 

In  the  pupa  stage  of  many  insects  we  find  remarkable 
special  resemblances.  Perhaps  the  finest  example  is  fur- 
nished by  the  cocoon  of  the  "waved-yellow  moth,"  Logoa 
opercularis.  The  pupa  of  this  moth  lies  inside  a  cocoon 
which  in  color  and  apparent  texture  closely  resembles  the 
bark  of  the  alder  and  other  twigs  on  which  it  is  found 
(Plate  63,  A\  At  the  top  of  the  cocoon  is  a  trap-door 
not  noticeable  until  it  opens  to  free  the  •adult  insect.  At 
the  middle  of  the  cocoon  there  is  a  peculiar  depression 
with  rough  elevated  edges,  giving  an  appearance  almost 
identical  with  that  of  the  winter  buds  of  the  alder  twigs. 
Another  species  of  the  same  genus  (L.  crispata)  has  a 
cocoon  of  quite  different  character  (Plate  63,  E\  for,  since 
it  is  found  underground,  there  is  no  need  of  its  having  the 
peculiarities  which  so  perfectly  protect  the  cocoon  of  L. 
opercularis.  The  caterpillars  of  these  same  moths  are 
also  protected  by  great  numbers  of  yellow  or  brown  hairs. 
In  L.  opercularis  the  hairs  so  completely  conceal  the  body 
of  the  caterpillar  that  one  would  not  suspect  its  real  nature 
(Plate  63,  B}.  In  L.  crispata  the  hairs,  while  present,  are 
less  thickly  set,  allowing  the  form  of  the  caterpillar  to  be 
seen  (Plate  63,  C  and  D].  Both  in  its  larval  stage  and  in 
the  chrysalis  L.  opercularis  is  more  perfectly  protected 
than  is  L.  crispata. 

The    examples    of    special    resemblance    thus    far    cited 


126 


ORGANIC  EVOLUTION 


have  all  been  taken  from  the   insects.     Examples  could  be 
found  in  other  groups.     Along  our  eastern  coast  is  a  small 


FlG.  31.  —  A  crab  (Cryptolithodes  sitchensis)  which  resembles  a  pebble.     Its  color  is  a  bluish 
gray,  resembling  a  piece  of  slate.  — From  a  specimen  collected  in  Puget  Sound. 

spider   found    very  frequently  on    the    little    roadside    rush, 
Juncus   bufonius,  which    so    closely  resembles    the    buds  of 

the  rush  in  color  and  shape 
that  the  most  careful  observer 
could  be  excused  for  not  detect- 
ing the  imposition  (Plate  64,  A\ 
Many  other  spiders  show  special 
protective  resemblances  (Plate 
64).  One  of  the  crabs  found  in 
Puget  Sound  is  so  exactly  like 
the  pebbles  of  the  bottom  along 
shore  that  no  one  would  recog- 
nize it  as  a  crab  until  he  saw  it 
in  motion  (Fig.  31).  In  the 
tufts  of  floating  seaweed,  so 
abundant  in  the  Sargassum  Sea, 
there  are  small  fishes  of  two 
FIG.  32. -A  "sea-horse-  (Hippocampus  species  which  in  color  are  pe- 

mohnikei),  a  fish  which  is  highly  modified  to  .. 

resemble  the  seaweed  attached  to  which  it  CUliarly    like    the    Seaweed     itself 

lives.     [After  JORDAN,  in  the  Proceedings  /ri1 

of  the  United  States  National  Museum.]  (1  Jate     65).  The      Seaweed      IS 


PLATE  64.  —  SPIDERS  WHOSE  COLOR  AND  SHAPE  RENDER  THEM  DIFFICULT  TO  SEE. 

A.  Epeira  stellata  upon  a  rush  (Juncus  bufonius),  natural  size;  from  a  specimen  given  by 
H.  W.  Britcher.  When  this  spider  rests  with  its  legs  folded,  its  resemblance  to  a  seed  pod  of  the 
rush  is  very  close.  B.  Ariamnes  attenuata,  which  resembles  a  stick.  [From  G.  W.  and  E.  G. 
PECKHAM,  after  CAMBRIDGE.]  C.  A  spider  which  resembles  a  seed  pod,  natural  size.  D  and 
E.  Cisrostris  mitralis,  which  resembles  a  knot  on  a  twig  (magnified).  [From  G.  W.  and  E.  G. 
PECKHAM,  after  VINSON.]  F.  Epeira  prompta  on  a  lichen-covered  branch.  [After  G.  W.  and 
E.  G.  PECKHAM.]  G.  Uloborus  plumipes,  with  its  cocoon  in  its  web  on  a  twig  of  larch.  [After 
G.  W.  and  E.  G.  PECKHAM.] 


PLATE  65. — SARGASSUM  FISH  (Plerophryne  histrio)  IN  A  TUFT  OF  FLOATING  SEAWEED. 

The  white  spots  on  the  fish  resemble  the  spots  of  Bryozoa  upon  the  seaweed.  The  fins  of  the 
fish  are  frayed  out  and  irregular,  resembling  somewhat  the  fronds  of  the  seaweed.  Two  pairs  of 
the  fins  are  modified  to  form  clasping  organs,  by  means  of  which  the  fish  clings  to  sprays  of  the 
seaweed. 


COLOR  IN  ANIMALS  12 7 

mottled  light  and  darker  brown  with  small  white  blotches, 
and  these  colors  are  reproduced  in  the  fishes  and  with  the 
characteristic  irregularity  seen  in  the  seaweed.  (See  also 
Fig-  32-)  Many  other  examples  might  be  cited,  but  enough 
has  been  said  to  emphasize  the  remarkable  nature  and 
the  prevalence  of  phenomena  of  protective  color  and  re- 
semblance. 

Aggressive  coloration  and  resemblance. 

Let  us  next  look  at  some  instances  of  aggressive  color- 
ation   and    resemblance.     Here    we    have    phenomena    very 


FIG.  33. — Tree-frogs  whose  backs  resemble  oak  leaves  in  color  and  color  pattern.     [From 
BEDUARD.] 

similar  to  those  just  illustrated,  but  the  use  of  the  color  or 
resemblance  is  just  the  opposite  to  that  which  we  have 
seen.  Instead  of  enabling  its  possessor  to  escape  its  enemies 
the  color  or  resemblance  enables  it  to  capture  its  prey. 
Anything  which  will  render  a  predaceous  animal  less  con- 


I2g  ORGANIC  EVOLUTION 

spicuous  will  aid  it  in  stalking  its  prey,  or,  as  it  lies  in  wait, 
to  capture  it.  Often  the  same  color  which  protects  an 
animal  from  its  own  enemies  will  also  aid  it  in  its  search 
for  food,  so  that  the  same  characters  will  be  both  protective 
and  aggressive.  The  dull  color  of  the  field  sparrow  (Plate 
49,  A)  will  enable  it  to  escape  the  view  of  the  hawk,  but 
also  it  will  enable  it  unobserved  to  approach  its  insect  prey. 
Many  of  the  color  characters  already  referred  to  probably 
have  this  double  use;  e.g.  think  of  the  insect-eating  birds  in 
general,  the  lizards  (Plate  52),  the  frogs  and  toads  (Fig.  33  and 

Plate  66),  the  snakes,  the 
leaf  mantis,  which  is  a  pre- 
daceous  form  feeding  upon 
small  insects  (Plate  62,  B] ; 
think  of  the  numerous  un- 
obtrusively colored  spiders 
(Plates  64  and  85,  D\  of  the 
pebble-like  crab  (Fig.  31), 
and  the  Sargassum  fish 
(Plate  65).  While  the  color 
of  the  animal  often  has  this 
double  significance,  there 
are  many  instances  in  which  the  color  is  purely  aggressive. 
To  this  class  belong  the  colors  of  the  polar  beary  white  like 
the  snow  (Fig.  34) ;  of  the  arctic  fox,  white  in  winter  and 
grayish  brown  in  summer  (Fig.  35)  of  the  weasel  (Plate  67) 
and  of  the  snowy-owl,  both  of  wh'  i  a  similar  seasonal 

change;  of  the  wolf,  the  fox,  th  I  ion-;  of  the  tiger,  tawny 
with  dark  stripes,  resembling  t  deal  shadows  of  the 

reeds  among  which  it  lies  ;n  wa  for  the  antelopes  as 
they  come  to  the  waterside  r  c  (Plate  68,  A};  of  the 


Fir,.  34.  —  Polar  bear  (Ursus  maritimus). — 
From  a  block  obtained  from  the  New  York  Zoo- 
logical Society. 


PLATE  66.—  A.  Common  toad  (Bufo  lentiginosus} .    B.  Tree-frog,  "tree-toad"  (Hylaversicolor), 
on  a  pine  tree.  —  From  a  photograph  obtained  from  the  New  York  Zoological  Society. 


PLATE  67.  —  \VEASELS  (Putorius  ermineus).- 

A.   In  winter.     B.   In  summer  pelage.  —  From  photographs  of  exhibits  in  the  American  Museum 
of  Natural  History.     Phoiographs  given  hy  the  Museum. 


COLOR   IN  ANIMALS 


I29 


jaguar,  a  forest  species  and  a  tree  climber,  the  blotches  on 
whose  skin  resemble  the  confused  shadows  among  the  trees 
(Plate  68,  B\ 


FIG.  35. — Arctic  fox,  in  winter  and  in  summer  pelage.     [After  BEDUARD.j 

Alluring  coloration  and  resemblances, 

There  are  a  few  examples  of  a  still  more  remarkable 
use  of  color  and  resemblance.  In  India  there  is  a  Mantis 
which  in  shape  and  color  resembles  an  orchid  blossom 
(Fig.  36).  It  deceives  butterflies  and  other  insects,  which 
it  captures  as  they  approach  the  seeming  flower.  In  Java 
there  is  a  spider  which  resembles  a  bit  of  bird-excrement 
upon  which  butterflies  are  so  apt  to  light.  This  resem- 
blance enables  it  to  capture  the  butterflies  upon  which  it 
feeds.  Forbes,  in  his  interesting  book,  A  Naturalist's  Wan- 
derings in  the  Eastern  Archipelago,  thus  describes  his 
discovery  of  this  peculiar  spider :  "  I  had  been  allured 
into  a  vain  chase  after  one  of  those  large,  stately  flitting 
butterflies  (Hestid)  through  a  thicket  of  prickly  Padanus 


130  ORGANIC  EVOLUTION 

horridus,  to  the  detriment  of  my  apparel  and  the  loss  of 
my  temper,  when  on  the  bush  that  obstructed  my  further 
pursuit  I  observed  one  of  the  Hesperidce  at  rest  on  a  leaf 
on  a  bird's  dropping.  I  approached  with  gentle  steps  but 
ready  net.  ...  It  permitted  me  to  get  quite  close  and 

even  to  seize  it 
between  my  fin- 
gers ;  to  my  sur- 
prise, however, 
part  of  the  body 
remained  be- 
hind, .  .  .  adher- 
ing, as  I  thought, 
to  the  excreta. 
I  looked  closely 
at,  and  finally 
touched  with  the 
tip  of  my  finger, 
the  excreta,  to 
find  if  it  were 
glutinous.  To 
my  delighted 
astonishment  I 
found  that  my 
eyes  had  been  most  perfectly  deceived,  and  that  the  excreta 
was  a  most  artfully  colored  spider  lying  on  its  back,  with 
its  feet  crossed  over  and  closely  adpressed  to  the  body. 

"  The  appearance  of  the  excreta  rather  recently  left  on 
a  leaf  by  a  bird  or  lizard  is  xvell  known.  Its  central  and 
denser  portion  is  of  a  pure  white  chalklike  color,  streaked 
here  and  there  with  black,  and  surrounded  by  a  thin 


FIG.  36.  — A  maims  {Hymenopusbicornis),  which  resembles  an 
archid  blossom.    By  courtesy  of  Crowell  Publishing  Company. 


COLOR   IN  ANIMALS  !3! 

border  of  the  dried-up  more  fluid  part,  which,  as  the  leaf  is 
rarely  horizontal,  often  runs  for  a  little  way  toward  the 
margin.  The  spider,  which  belongs  to  a  family,  the  Tho- 
misidcz,  possessing  rather  tuberculated,  thick,  and  prominent 
abdomened  bodies,  is  of  a  general  white  color ;  the  underside, 
which  is  the  one  exposed, -is  pure  chalk-white,  while  the  lower 
portions  of  its  first  and  second  pairs  of  legs  and  a  spot  on 
the  head  and  on  the  abdomen  are  jet  black. 

"  This  species  does  not  weave  a  web  of  the  ordinary  kind, 
but  constructs  on  the  surface  of  some  prominent  dark  leaf 
only  an  irregularly  shaped  film,  of  the  finest  texture,  drawn 
out  toward  the  sloping  margin  of  the  leaf  into  a  narrow 
streak,  with  only  a  slightly  thickened  termination.  The  spi- 
der then  takes  its  place  on  its  back  on  the  irregular  patch  I 
have  described,  holding  itself  in  position  by  means  of  several 
strong  spines  on  the  upper  sides  of  the  thighs  of  its  anterior 
pair  of  legs  thrust  under  the  film,  and  crosses  its  legs  over  its 
thorax.  Thus  resting  with  its  white  abdomen  and  black  legs 
as  the  central  and  dark  portions  of  the  excreta,  surrounded 
by  its  thin  web-film  representing  the  marginal  watery  portion 
become  dry,  even  to  some  of  it  trickling  off  and  arrested  in  a 
thickened  extremity  such  as  an  evaporated  drop  would  leave, 
it  waits  with  confidence  for  its  prey,  —  a  living  bait  so  artfully 
contrived  as  to  deceive  a  pair  of  human  eyes  even  intently 
examining  it." 

In  Algiers  is  found  a  lizard  which  has  at  the  corners  of  its 
mouth  bright  red  folds  of  skin  which  are  of  the  same  color  and 
shape  as  the  blossoms  of  one  of  the  desert  plants.  Insects 
are  deceived  and  come  to  feed  upon  the  nectar  and  pollen,  but 
serve  themselves  as  food  for  the  lizard.  These  are  examples 
of  what  we  may  call  alluring  coloration  and  resemblance. 


132 


ORGANIC  EVOLUTION 


IV a  rn  ing  color  a  tion . 

Warning  colors  are  another  important  class.     Many  ani- 
mals are  dangerous  because  of  some  means  of  defence,  or  are 
noxious  or  nauseous  as  food,  and   many  such   are   conspic- 
uously colored,  as  if  advertising  their  dangerous  or  disagree- 
able nature.     Many  insects  show  conspicuous  colors  of  this 
sort.     Many  of  the  bees,  wasps,  hornets,  and  yellow-jackets 
are  conspicuously  banded  with  yellow  or  white,  or  have  a 
brilliant    metallic    lustre,   like    the    blue    wasps    (Plate    74). 
That  this  conspicuous  coloration  is  an  actual  protection  to 
these  stinging  insects  is  readily  shown  by  experiment.     Very 
few  insect-eating  birds,  lizards,  frogs,  toads,  or  mammals  will 
eat  these  insects.     Apparently  they  have  learned  that  they 
are  unpalatable.     By  experimenting  with  young  birds  which 
have  never  before  seen  bees  or  wasps  we  get  evidence  that 
the  noxious  character  of  the  insect  has  to  be  learned,  but  it 
is  learned  with  astonishing  rapidity,  and  when  once  learned, 
seems  not  to  be  forgotten.     Lloyd   Morgan  describes  feed- 
ing  a   young   chick   with    flies  among    which   he    placed    a 
wasp.     The  chick   took  the   wasp,   was  stung,   and   showed 
great  agitation,   wiping  its  bill   and   scratching  it.     Several 
days  later,  while  again  feeding  the  little  fellow  with  flies,  he 
offered    it   another  wasp.     The   chick   looked    at    the   wasp, 
turned  away  from  it,  and  began  wiping  its  bill,  apparently 
remembering  the  disagreeable  sensations  which  followed  its 
former  attempt    to  eat  a  wasp.     Hundreds   of  experiments 
show  a  similar  ability  in  other  birds,  in  lizards,  frogs,  toads, 
and  monkeys,  to  rapidly  learn  the  unpalatable  character  of 
conspicuous    insects.      If   the    stinging   Hymcnoptera^    were 
less    conspicuously  colored,    they  would   often   be    mistaken 
for  edible  forms  and  either  be  eaten  or  at  least  be  grasped 

1  Ants,  bees,  and  wasps. 


-^ 


PLATE  69.  —  A.  Two  bugs  (Prionotus  cristatus  on  the  left  and  Euchistus  serTiis  on  the  right) 
whose  odor  and  flavor  are  disagreeable  to  insect-eating  birds,  lizards,  toads,  and  frogs,  and  whose 
shape  is  easily  recognized,  causing  them  to  be  avoided.  B.  Lady  beetles  (Hippodamia  convergent, 
Megilla  maculata,  Adalia  bipunctata}.—  ^>\  the  courtesy  of  the  United  States  Department  of  Agri- 
culture. C.  Colorado  potato  beetle  (Doryphnra  decemlineata);  a,  eggs;  b,  larva;  c,  adult.  —  By 
the  courtesy  of  the  United  States  Department  of  Agriculture. 


COLOR  IN  ANIMALS  133 

and  injured,  even  if  finally  rejected  without  being  eaten. 
Their  conspicuous  color  is  readily  remembered,  and,  as  it  is 
associated  in  the  minds  of  their  enemies  with  their  dis- 
agreeable character,  it  must  serve  to  save  many  from  injury 
or  destruction.  The  coloration,  therefore,  is  properly  called 
warning  coloration. 

Often  such  warning  coloration  is  associated  with  peculiar 
shape  or  distinctive  habits  which  make  the  insect  still  more 
easy  to  recognize.  The  bees,  wasps,  yellow-jackets,  and  hor- 
nets have  a  peculiar  buzzing  flight,  and  when  standing,  they 
commonly  teeter  the  abdomen  up  and  down  in  a  way  that 
always  suggests  to  us  their  excitable  disposition.  Apparently 
these  habits  produce  much  the  same  effect  upon  their  bird, 
lizard,  and  frog  enemies  that  they  do  upon  us.  The  slender 
waist  of  the  Hymenoptera  is  also  a  conspicuous  feature. 

As  further  examples  of  warning  coloration  we  might  call 
attention  to  the  Hemiptera,  the  bugs,  many  of  which  have  a 
very  disagreeable  taste  and  equally  disagreeable  odor.  These 
insects  are  frequently  conspicuously  colored,  and  they  gener- 
ally have  a  very  characteristic  and  readily  recognized  body 
form  (Plate  69,  A).  Many  of  the  beetles  are  very  tough  and 
some  are  disagreeable  in  flavor;  accordingly  we  find  many 
conspicuously  colored  beetles.  Perhaps  the  best  example  is 
the  common  Colorado  potato-beetle,  the  adult  of  which  is  con- 
spicuously marked  with  longitudinal  stripes  and  whose  larva 
is  also  bright-colored  and  conspicuously  spotted  (Plate  69,  C). 
Both  the  adults  and  the  larvae  are  unpalatable  to  birds,  lizards, 
frogs,  and  toads.  Other  examples  among  the  beetles  are  the 
goldenrod-beetle  and  the  lady-beetles,  commonly  miscalled 
ladybugs  (Plate  69,  B}.  Many  conspicuously  colored  butter, 
flies  are  inedible ;  for  example,  the  common  yellow  and  white 


I34  ORGANIC  EVOLUTION 

forms,  Pierida,  found  so  frequently  about  wet  places  in  the 
roads  (Plate  59,  A\  and  most  of  the  swallow-tailed  butterflies, 
Papilionida,  which  are  our  most  conspicuous  North  American 
forms  (Plate  76,  D\  Some  moths  show  warning  color  (Plate 
70,  A-K\  The  larvae  of  many  moths  and  butterflies  are 
inedible,  and  these  also  are  conspicuously  colored  (Plate  71). 

Wallace,  in  his  Darwinism,  says :  "  These  uneatable 
insects  are  probably  more  numerous  than  is  supposed, 
although  we  already  know  immense  numbers  that  are  so 
protected.  The  most  remarkable  are  the  three  families  of 
butterflies  —  Heliconidcz  [Plate  77,  A-D~\,  Danaidc?  [Plate 
76,  A,  E,  and  Plate  84,  E  and  F],  and  Acraida  [Plate  76, 
G,  /,  and  77,  /,  L~\  —  comprising  more  than  a  thousand  spe- 
cies, and  characteristic  respectively  of  the  three  great  tropical 
regions:  South  America,  Southern  Asia,  and  Africa.  All 
these  butterflies  have  peculiarities  which  serve  to  distinguish 
them  from  every  other  group  in  their  respective  regions. 
They  all  have  ample  but  rather  weak  wings,  and  fly  slowly. 
They  are  always  very  abundant;  and  they  all  have  con- 
spicuous colors  or  markings,  so  distinct  from  those  of  other 
families  that,  in  conjunction  with  their  peculiar  outline  and 
mode  of  flight,  they  can  usually  be  recognized  at  a  glance. 
Other  distinctive  features  are,  that  their  colors  are  always 
nearly  the  same  on  the  under  surface  of  their  wings  as  on  the 
upper ;  they  never  try  to  conceal  themselves,  but  rest  on  the 
upper  surfaces  of  leaves  or  flowers ;  and,  lastly,  they  all  have 
juices  which  exhale  a  powerful  scent,  so  that  when  one  kills 
them  by  pinching  the  body,  the  liquid  that  exudes  stains  the 
fingers  yellow,  and  leaves  an  odor  that  can  only  be  removed 
by  repeated  washings. 

"  Now  there  is  much  direct  evidence  to  show  that  this 


PLATE  70.  —  WARNING   COLORATION  AND  MIMICRY  IN  MOTHS.     [After  KAPPEL  AND  KiRBY.J 

A-K.  Inedible  moths,  showing  warning  coloration.  A.  Zygcena  trtfolii.  B.  Callimorpha 
dominula.  C.  Zygcena  epialtes.  D.  Emydiajacobece.  E.  Callimorpha  hera.  F.  Zygtena  achillece. 
G.  Zygcena  minor .  H.  Arctia  caja.  I.  Zygcena  fausta.  J.  Zenzera  cesculi.  K.  Abraxas  glos- 
sulariata.  L-O.  Mimicry  of  bees  and  wasps  by  moths.  L.  Sesia  culiciformis.  M.  Sesiatipuh- 
formis.  N.  Trochilium  apiforme.  O.  Macroglossia  bombyliformis.  P-S.  Moths  closely  related 
to  L-O,  which  do  not  imitate  bees  or  wasps.  P,  Macroglossia  stellatarum ;  cf.  O.  Q.  Pterogoti 
proserpina.  R.  Ino  pruni.  S.  hio  statices. 


PLATE  71.  — INEDIBLE  CATERPILLARS,  SHOWING  WARNING  COLORATION.     [After  KAPPEL 
AND  KIRBY.] 

A.  Papillio  machaon.     B.  Arctia  caja.    C.  Orgya  antiqua.     D.  Acronycta  alni.    E.  Acronycta 
psi.    F.  Parnassins  apollo.     G.  Melitaa  cinxia.     H.  Leucoma  solids. 


PLATE  72. —  A.  Gila  monster  {Heloderma  horriduni}.  B.  Som»  varieties  of  North  American 
skunks  of  the  sub-genus  Chincha  :  I.  Chincha  mesomelas  (Louisiana)  ;  2.  C.  mephitis  (Keewatin)  ; 
3.  C.  estor  (Arizona);  4.  C,  putida  (Massachusetts);  5.  C.  elongata  (Florida);  6  and  7. 
C.  macroura  milleri  (Northern  Mexico).  Figures  of  skunks  from  A.  H.  Howell's  Revision  of 
the  Sk^lnksofthe  Genus  Chincha  {North  American  Fauna,  No.  20),  by  the  courtesy  of  the  United 
States  Department  of  Agriculture. 


COLOR   IN  ANIMALS 


135 


odor,  though  not  very  offensive  to  us,  is  so  to  most  insect- 
eating  creatures.  Mr.  Bates  observed  that,  when  set  out  to 
dry,  specimens  of  Heliconida  were  less  subject  to  the  attacks 
of  vermin ;  while  both  he  and  I  noticed  that  they  were  not 
attacked  by  insect-eating  birds  or  dragon-flies,  and  that 
their  wings  were  not  found  in  the  forest  paths  among  the 
numerous  wings  of  other  butterflies  whose  bodies  had  been 
devoured." 

Among  the  Amphibia  the  frogs  are  edible  and  are  pro- 
tectively colored.      Toads  are  distasteful,    but  show  a  dull 
color  which  is  probably  aggressive,  aiding  them  in  capturing 
their  insect  prey  (Plate 
66,  B}.     The   salaman- 
ders, on  the  other  hand, 
are    night    feeders    and 
do   not  need   to  be  ag- 
gressively  colored,    and 
we  frequently  find  them 
very      conspicuously 
spotted,   since  they  are 
inedible  (Fig.  37). 

Lizards,  almost  without  exception,  show  dull  colors,  or 
colors  that  are  in  harmony  with  their  environment,  their  col- 
oration being  both  protective  and  aggressive  (Plate  52).  It 
is,  therefore,  especially  interesting  to  find  that  the  only  known 
poisonous  lizard,  the  Gila  monster  of  our  southwestern  states, 
is  a  conspicuously  colored  form,  salmon-pink  with  broad 
irregular  black  bands  and  blotches  (Plate  72,  A\ 

The  Mammalia  as  a  rule  show  aggressive  or  protective 
coloration  in  harmony'  with  their  surroundings ;  the  skunk, 
however,  which  is  so  effectively  protected  by  the  foul-smelling 


FIG.  37.  —  Salamander  (Salama/idra  maculosa). — 
From  Brehm's   Thierleben. 


I36  ORGANIC  EVOLUTION 

secretion  of  its  scent  glands,  advertises  its  disagreeable  char- 
acter by  its  conspicuous  black-and-white  color  (Plate  72,  B\ 
There  are  a  number  of  similar  instances  among  the  Mam- 
malia. The  black-and-white  color  of  the  skunk  probably 
renders  it  inconspicuous  when  hunting  its  prey  on  moonlight 
nights,  the  black  resembling  shadows,  and  the  white  marks 
blotches  of  light.  Its  color,  therefore,  is  probably  both 
aggressive  and  warning  coloration,  aggressive  by  night, 
warning  by  day. 

Similar  phenomena  of  warning  coloration  are  found 
among  the  different  groups  of  marine  invertebrates,  but, 
as  the  forms  are  less  familiar,  we  will  not  refer  to  them. 

Convergence  in  warning  coloration. 

One  very  interesting  feature  is  observed  in  the  warning 
coloration  of  the  inedible  butterflies.  Different  inedible 
species,  belonging  to  distinct  genera  or  even  to  distinct 
families,  in  many  instances  show  the  closest  similarity  in 
color  and  in  color  pattern,  and  often  also  in  shape  (Plates  76 
and  77,  A-F}.  This  was  for  a  long  time  a  puzzle  to  stu- 
dents of  color  phenomena,  until  the  German  naturalist, 
Fritz  Miiller,  suggested  that  this  convergence  in  coloration 
among  unrelated  inedible  butterflies  must  decrease  consider- 
ably the  number  of  experiments  necessary  to  teach  young 
birds  and  lizards  the  evil  character  of  the  butterflies,  since 
they  are  all  of  one  pattern,  and  so  save  from  destruction 
many  individuals  which  would  be  sacrificed  did  their  enemies 
need  to  learn  a  separate  pattern  for  each  inedible  species. 
This  suggestion  seems  plausible.  It  is,  at  least,  the  best  we 
have  yet  found. 


PLATE  73.—  A.  Inedible  curculios  and  lady-beetles  imitated  by  edible  longicorn  beetles  and 
grasshoppers.  All  from  the  Philippine  Islands. 

a.  Doliops  sp.,  edible  longicorn  beetle  which  imitates  b.  Pachyrhynchus  orbifts,  a  hard  curculio. 
c.  Doliops  cnrculionides,  a  longicorn  beetle  which  imitates  d.  Pachyrhynchus  sp.,  a  hard  curculio. 
e.  Scepastus  pachyrhynchoides,  a  grasshopper  which  imitates  /.  Apocyrtus  sp.,  a  hard  curculio. 
g.  Doliops  sp.,  a  longicorn  beetle  which  imitates  A.  Pachyrhynchus  sp.,  a  hard  curculio.  i.  Pho- 
raspis,  a  grasshopper  which  imitates  k.  CoccinelJa,  an  inedible  lady-beetle.  [From  WALLACE.] 
The  resemblance  is  exact  in  color  as  well  as  color  pattern. 

B.  A  wasp  (a.  \fygnimia  aviculus)  which  is  imitated  by  a  longicorn  beetle  (b.  Coloborhombus 
fasciatipennis) .  [From  WA  LLAC  E.] 


3,4 


9,  10 


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t 


iWHMM«BPHMMMiMBiMMMHBMBiMMMMRHHi^mMMi 

PLATE  74. —  SEVERAL  SPECIES  OF  FLIES,  AND  THE  BEES  AND  WASPS  WHICH  THEY 
RESEMBLE. 

I.  Mydas  clavatus.  2.  Pompilus  atrox.  3  and  5.  Apis  mellifera.  4  and  6.  Eristalis  tenat. 
•j.  Spilomyia  hamifera.  8.  Vespa  occidentalis.  9-11.  Bombus  vancouverensis.  12-14.  I'olucella 
facialis. 


COLOR  IN  ANIMALS  137 

Immunity  coloration. 

Reighard  has  called  in  question  the  interpretation  of 
brilliant  colors  as  warning  colors.  In  his  work  upon  the 
fishes  of  the  Tortugas  he  found  that  the  conspicuously 
colored  fish  were  confined  to  the  coral  reefs,  and  that  the 
fish  upon  the  sand  bottom  were  protectively  colored.  He 
found  that  the  gray  snappers  —  the  chief  predatory  fish  of 
this  region  —  ate  readily  and  repeatedly  the  brilliant  colored 
coral  reef  fishes  when  they  were  taken  from  the  reefs  and 
were  thrown  into  shallow  water  above  a  sand  bottom  where 
of  course  they  were  very  conspicuous.  Reighard  believes  that 
the  reef  fishes  are  safe  from  attack  by  predatory  enemies 
because  of  the  abundance  of  safe  hiding  places  in  their 
habitat,  the  caves  and  channels  among  the  corals  enabling 
them  to  escape  pursuit.  He  thinks  this  safety  from  attack 
allows  them  to  become  conspicuously  colored  without  ex- 
posing them  to  destruction,  and  that  their  conspicuousness 
is  due  to  this  immunity.  He  has  used  the  term  immunity 
coloration  for  such  conspicuous  color  of  species  that  are  free 
from  attack.  He  believes  that  most  conspicuous  coloration 
in  animals  is  immunity  coloration. 

That  conspicuous  coloration  has  generally  developed  as 
immunity  coloration  seems  not  improbable,  but  there  seems 
little  doubt  that  when  once  developed  it  serves  in  many 
species  as  warning  coloration.  The  conspicuously  colored 
species  that  are  inedible  or  noxious  are  easily  recognized  by 
predatory  forms  and  are  avoided. 

Mimicry. 

Some  of  the  instances  of  protective,  alluring,  and  warning 
coloration  that  have  been  described  are  sufficiently  remark- 


I38  ORGANIC  EVOLUTION 

able,  but  the  phenomena  of  mimicry  are  even  more  surpris- 
ing. Many  animals  which  are  not  protected  by  stings,  or 
disagreeable  odors  or  flavors,  and  are  really  palatable  to 
predaceous  species,  are  protected  from  the  attacks  of  such 
predaceous  enemies  by  their  resemblance  to  species  which 
are  inedible.  Instances  of  such  mimicry  are  very  numerous 
among  the  insects,  and  are  found  also  in  other  groups.  Let  ' 
us  see  some  examples. 

Many  beetles  are  inedible,  either  because  of  their  very 
hard  outer  shell,  or  because  of  some  nauseous  flavor,  and  we 
find  many  such  forms  to  be  conspicuously  marked  with 
'strongly  contrasted  colors;  e.g.  the  lady-beetles  and  curculios 
(Plate  73,  A,  b,  d,  f,  h,  k).  There  are  edible  beetles  which 
mimic  some  of  these  warning-colored  inedible  forms  (Plate  73, 
A,  a,  c,  g).  The  hard  and  unpalatable  curculios  are  imitated 
also  by  grasshoppers  (Plate  73,  A,  e).  Certain  grasshoppers 
also  imitate  the  evil-flavored  lady-beetles  (Plate  73,  A,  i). 

Wasps,  bees,  hornets,  and  yellow-jackets  are  armed  with 
stings  which  make  them  dangerous  to  attack,  and  their  dan- 
gerous character  is  usually  advertised  by  their  conspicuous 
coloration.  As  we  would  naturally  expect,  we  find  that  they 
are  frequently  imitated  by  other  insects.  We  have  longi- 
corn  beetles  which  mimic  wasps  (Plate  73,  B}.  Very  many 
flies  mimic  bees  and  wasps  (Plate  74).  One  common  kind  of 
fly  imitates  the  honey-bee  so  closely  that  one  would  hesitate 
to  handle  it  even  after  being  told  that  it  is  harmless.  Other 
flies  mimic  bumble-bees  in  appearance  and  in  manner  of 
flight.  In  all  of  these  cases,  the  resemblance  is  enhanced  by 
the  habits  of  the  imitating  form.  The  drone-fly,  for  example, 
which  imitates  a  honey-bee,  has  the  same  kind  of  buzzing 
flight,  and,  when  standing,  occasionally  teeters  its  abdomen 


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COLOR  IN  ANIMALS  139 

up  and  down,  as  is  characteristic  of  the  bees  and  wasps. 
Some  of  these  mimicking  flies  even  protrude  and  withdraw 
the  tip  of  the  abdomen,  as  does  an  angry  bee  or  wasp, 
making  the  imitation  in  habit  as  well  as  in  form  and  color 
as  perfect  as  possible. 

At  Wood's  Holl  one  summer,  while  collecting  insects 
from  the  blossoms  of  the  common  milkweed,  I  was  struck  by 
the  resemblance  of  a  moth  to  the  large  metallic  blue  wasp. 
When  the  moth  was  at  rest  upon  the  milkweed  blossoms, 
this  resemblance  was  not  marked,  but  as  one  approached  at 
all  near,  the  moth  sprang  into  the  air,  flying  with  a  peculiar 
buzzing  flight  that  seemed  at  once  to  transform  it  into  a 
\\Y.sp.  The  blue  wasps  were  common  upon  the  same  blos- 
soms, and  the  deception  was  very  perfect.  As  these  moths 
are  keen-sighted  and  easily  startled,  they  must  rarely  be  cap- 
tiired  while  at  rest,  and  when  flying  they  are  likely  to  be  let 
alone  by  insect-eating  birds  and  dragon-flies.  In  the  Alle- 
ghany  Mountains  I  have  found  a  large,  blue-back  longicorn 
beetle,  which  when  in  flight  closely  resembles  one  of  the  blue 
wasps.  We  have  an  American  moth  which  similarly  resem- 
bles a  bumble-bee,  only  in  this  case  the  resemblance  is  almost 
as  noticeable  when  the  moth  is  at  rest  as  when  it  is  in  flight 
(Plate  70,  O).  The  body  has  the  same  shape,  is  banded  with 
yellow,  and  is  covered  with  similar  long  yellow  hairs ;  the 
wings  also  are  very  different  from  those  of  most  moths, 
having  lost  most  of  their  scales  and  being  transparent,  like 
the  wings  of  a  bumble-bee.  Many  other  moths  mimic  the 
stinging  Hymenoptera  (Plate  70,  L,  M,  N). 

The  ants,  another  group  of  the  Hymenoptera,  'are  hard, 
gritty  little  insects,  with  an  acid  flavor,  and  are  not  esteemed 
as  food  by  insect-eating  birds.  Some  even  have  stings,  like 


140 


ORGANIC  EVOLUTION 


their  relatives  the  bees  and  wasps.  In  the  tropics  certain 
species  of  ants  are  in  the  habit  of  gathering  bits  of  leaves 
from  the  trees  and  taking  them  to  their  nests  to  fertilize 
their  fungus  gardens.  These  leaf-cutting  ants  are  often 
seen  in  great  abundance,  marching  in  procession  from  the 
tree  which  is  being  denuded  to  their  nest,  each  with  a  piece 
of  green  leaf  held  in 
his  jaws  and  hang- 
ing back  over  his 
shoulder.  Among 
some  of  these  leaf- 
cutting  ants  in  the 

Amazon      basin,      Mr.  FIG.  38._An  ant  (a)  which  in  size,  spread  of  legs,  glossy 

T7nrr1icV)  b'ack  character  of  abdomen,  and  in  general  appearance  at  a 

I,      c             ^IlgUS  little  distance,  is  imitated  by  a  spider  (£)  which  lives  in  the 

list-         fYh^PrvPrl  same  nest-     Botn  are  quite  small.     It  is  very  difficult  for  one 

.1SI,            UbCl  VCU  observmg  tnem  closely  to  detect  the  spiders  among  the  ants. 

an  insect  belonging  - From  specimens  sent  by  H'w-  Britchen 
to  a  different  order,  a  "  tree-hopper,"  one  of  the  Homoptera, 
which  mimicked  the  ant  with  its  leaf  (Plate  75,  B\  Its 
body  was  brown  below,  like  the  ant,  and  above  was  drawn 
up  into  a  narrow  longitudinal  ridge,  irregular  in  outline  on 
the  upper  edge  and  colored  a  bright  green,  giving  the  whole 
insect  almost  the  exact  appearance  of  an  ant  carrying  a  bit 
of  green  leaf.  The  ants  being  unpalatable,  the  bug  which 
imitated  them  was  protected  from  attack  by  insect-eating 
birds.  Ants  are  also  mimicked  by  many  other  insects  and 
by  spiders  (Figs.  38  and  39). 

Many  species  of  edible  or  imperfectly  protected  butter- 
flies imitate  the  appearance  of  some  of  the  most  ill -flavored 
butterflies.  One  of  the  best  examples  is  found  throughout 
the  whole  of  eastern  North  America.  The  color  and  color 
pattern  of  the  inedible  Danais  archippus  is  imitated  by  the 
less  protected  Limenitis  disippus  (Plate  76,  A,  B,  C}.  I 


COLOR   IN  ANIMALS  !^! 

have  several  times  found  these  two  butterflies  flying  to- 
gether, and  the  first  time  I  captured  any  of  them  I  did  not 
see  until  I  reached  home  that  I  had  two  species,  instead  of 
one  as  I  thought  The  edible  form  is  slightly  smaller  than 
the  ill-flavored  one,  so  that  when  once  distinguished  they  can 
again  be  recognized 
without  difficulty,  but  I 
much  doubt  if  our  in- 
sect-eating birds  would 
detect  the  difference. 
The  inedible  He lie o- 
nidce  of  South  and 
Central  America  are 
imitated  by  less  pro-  a  b 

FIG.  39.  —  Spiders  which  mimic  ants. 

teC ted    SpecieS    Of    Other  a.   Synageles  picata.    b.   Synemosyna  formica.     [From 

r         .,.  ,-T.,     .  /-^      G.  W.  and  E.  G.  PECKHAM.] 

families    (Plate    77,   G, 

H,  B}.  The  highly  inedible  Acr&idce  of  Africa  are  imitated 
by  other  butterflies  (Plate  77,  L,  M).  One  of  the  most  re- 
markable cases  of  mimicry  is  that  of  the  imitation  of  three 
different  inedible  species  by  three  varieties  of  females  in  the 
less  distasteful  though  somewhat  protected  Papilio  merope 
(Plate  76,  D-J}.  As  Papilio  werope  is  itself  distasteful,  it 
might  be  better  to  call  these  conditions  an  illustration  of  con- 
vergence in  warning  coloration.  Perhaps  the  same  might  be 
said  of  all  mimicry  among  butterflies,  for  butterflies  have  few 
predacious  enemies. 

Euplcea  midamus,  an  inedible  butterfly,  is  mimicked  by 
Calamesia  midama,  a  moth  (Plate  84,  C,  D,  E,  F\  The  male 
and  female  butterfly  differ  in  color  and  in  the  pattern  of 
their  markings,  and  it  is  interesting  to  see  that  the  male 
moth  imitates  the  male  butterfly  and  the  female  moth  copies 
the  female  butterfly. 


I42  ORGANIC  EVOLUTION 

There  are  instances  in  which  insects  are  supposed  to  be 
protected  by  an  apparent  resemblance  to  certain  vertebrates. 
Let  me  quote  from  Professor  Poulton's  delightful  book  The 
Colors  of  Animals. 

"  Mr.  Bates  describes  a  South  American  caterpillar  which 
startled  him,  and  every  one  to  whom  he  showed  it,  by  its 
strong  resemblance  to  a  snake,  and  it  even  possessed  the 
features  which  are  characteristic  of  a  poisonous  serpent. 

"  Equally  interesting  examples  are  to  be  found  among 
our  British  caterpillars.  The  brown  (or  occasionally  green) 
mature  larva  of  the  large  elephant  hawk  moth  (Chrao- 
campa  elpenor]  generally  hides  among  the  dead  brown  leaves 
on  the  under  parts  of  the  stem  of  its  food-plant,  the  great 
willow  herb  (Epilobium  hirsutum)  (Plate  78,  A).  In  this 
position  it  is  difficult  to  see,  for  it  harmonizes  well  with  the 
color  of  its  surroundings.  It  possesses  an  eyelike  mark  on 
each  side  of  two  of  the  body  rings  (the  first  and  second 
abdominal  segments),  but  these  markings  do  not  attract 
special  attention  when  the  animal  is  undisturbed. 

"  As  soon,  however,  as  the  leaves-  are  rustled  by  an 
approaching  enemy,  the  caterpillar  swiftly  draws  its  head  and 
the  first  three  body  rings  into  the  next  two  rings,  bearing 
the  eyelike  marks.  These  two  rings  are  thus  swollen  and 
look  like  the  head  of  an  animal  upon  which  four  enormous, 
terrible-looking  eyes  are  prominent  (Fig.  40).  The  effect  is 
greatly  heightened  by  the  suddenness  of  the  transformation, 
which  endows  an  innocent  looking  and  inconspicuous  animal 
with  a  terrifying  and  serpentlike  appearance.  I  well  remem- 
ber the  start  with  which  I  drew  back  my  hand  as  I  was  going 
to  take  the  first  specimen  of  this  caterpillar  I  had  ever  seen." 

A  good  many  different  species  of  caterpillar  show  "  terri- 
fying "  attitudes  and  motions.  Poulton  thus  describes  the 


SiS  2-Q-o      ;S 


. 

-8    .„«•=§        *-0 

«l!i:5  si 


9!?fiia 

' 


-*-    -    g^ 


PLATE  77- — A-D.— Four  inedible  butterflies  belonging  to  four  different  genera  and  three  different 
families,  but  all  showing  the  same  type  of  warning  coloration  ;  an  example  of  convergence  in  warning 
coloration.  E-F.  I- K and  I- M  also  show  convergence  in  warning  coloration.  G  //illustrate  mimicry. 
I  After  WEISMANN.  with  modifications.] 

A.  Lycorea  ha.Ua,  inedible.  B.  Heliconiuseucrate,  inedible.  C.  Melina-a  ethra,  inedible.  D.  Mucha- 
nitis  lysfmnia.  inedible.  E.  Perhybris  pyrrha  (male),  which  shows  but  slight  resemblance  to  the 
ined'  le  Heliconidae.  F.  Perhybris  pyrrha  (female),  which  closely  resembles  the  Heliconidae.  G.  and 
//.  Male  and  female  Dismorphia.  astynome,  edible  ;  both  sexes  imitate  the  Heliconidw.  /.  Acreea  egina, 
of  the  tarnily  Acrasidae.  J  Papih'o  ridleyanns  Tfemale),  of  the  family  Papilionidae.  K.  Pseudacrtra 
bnisdnvalii,  of  the  family  Nymphalidse.  J  and  K,  which  are  inedible,  depart  from  the  type  of  colora- 
tion charcteristic  of  their  families  and  resemble  the  inedible  Acrsa  (  /).  L.  Acrcra  gea.  inedible,  which 
is  imitated  by  M.  Elymnias  pliegta,  an  inedible  species. 


COLOR  IN  ANIMALS  143 

behavior  of  the  caterpillar  of  the  puss  moth :  "  The  larva 
of  the  puss  moth  (Cerura  vinuld]  is  very  common  upon  pop- 
lar and  willow.  The  circular  domelike  eggs  are  laid  either 
singly  or  in  little  groups  of  two  or  three,  upon  the  upper 
side  of  the  leaf,  and  being  of  a  reddish  color  strongly  suggest 
the  appearance  of  little  galls  or  the  results  of  some  other  in- 
jury. The  youngest  larvae  are  black,  and  also  rest  upon  the 
upper  surface  of  the  leaf,  resembling  the  dark  patches  which 


FlG.  40.  —  Caterpillar  of  the  large  elephant  hawk-moth  {Cheer ocampa  elpenor).     [After  WEIS- 
MANN  and  POULTON.] 

a.  In  normal  position  when  feeding,     b.  In  "terrifying  attitude."     Compare  Plate  79,  Fig.  A, 
which  shows  the  same  caterpillar  in  natural  colors. 

are  commonly  seen  in  this  position.  As  the  larva  grows,  the 
apparent  black  patch  would  cover  too  large  a  space,  and 
would  lead  to  detection  if  -it  still  occupied  the  whole  surface 
of  the  body.  The  latter  gains  a  green  ground-color  which 
harmonizes  with  the  leaf,  while  the  dark  mark  is  chiefly  con- 
fined to  the  back.  As  growth  proceeds  the  relative  amount 
of  green  increases,  and  the  dark  mark  is  thus  prevented  from 
attaining  a  size  which  would  render  it  too  conspicuous.  In 
the  last  stage  of  growth  the  green  larva  becomes  very  large, 


144  ORGANIC  EVOLUTION 

and  usually  rests  on  the  twigs  of  its  food-plant.  The  dark 
color  is  still  present  on  the  back  but  is  softened  to  a  purplish 
tint,  which  tends  to  be  replaced  by  a  combination  of  white 
and  green  in  many  of  the  largest  larvae  (Plate  78,  D\  Such 
a  larva  is  well  concealed  by  general  protective  resemblance, 
and  one  may  search  a  long  time  before  finding  it,  although 
assured  of  its  presence  from  the  stripped  branches  of  the 
food-plant  and  the  fceces  on  the  ground  beneath. 

"  As  soon  as  the  larva  is  discovered  and  disturbed  it  with- 
draws its  head  into  the  first  body  ring,  inflating  the  margin, 
which  is  of  a  bright  red  color.  There  are  two  intensely 
black  spots  on  this  margin  in  the  appropriate  position  for 
eyes,  and  the  whole  appearance  is  that  of  a  large  flat  face 
extending  to  the  outer  edge  of  the  red  margin  (Plate  78,  D}. 
The  effect  is  an  intensely  exaggerated  caricature  of  a  verte- 
brate face,  which  is  probably  alarming  to  the  vertebrate  ene- 
mies of  the  caterpillar.  The  terrifying  effect  is  therefore 
mimetic.  The  movements  entirely  depend  upon  tactile 
impressions:  when  touched  ever  so  lightly  a  healthy  larva 
immediately  assumes  the  terrifying  attitude,  and  turns  so  as 
to  present  its  full  face  toward  the  enemy ;  if  touched  on  the 
other  side  or  on  the  back  it  instantly  turns  its  face  in  the 
appropriate  direction. 

"The  effect  is  also  greatly  strengthened  by  two  pink 
whips  which  are  swiftly  protruded  from  the  prongs  of  the  fork 
in  which  the  body  terminates.  The  end  of  the  body  is  at 
the  same  time  curved  forward  over  the  back  (generally  much 
further  than  in  the  figure),  so  that  the  pink  filaments  are 
brandished  above  the  head." 

Experiment  showed  that  the  terrifying  attitude  and  mo- 
tions were  effective  in  frightening  away  enemies.  I  suspect 


COLOR  IN  ANIMALS 


145 


that  the  suddenness  of  the  change  from  one  condition  to  the 
other  when  irritated  has  as  much  to  do  with  scaring  away 
enemies  as  does  the  reputed  resemblance  to  the  front  part 
of  a  snake,  for  most  insect-eating  birds  and  lizards  are  very 
wary  and  easily  startled. 

This  description  is  quoted  in  full,  for  it  gives  a  remark- 
able instance  of  the  combination  of  general  protective 
resemblance,  terrifying  attitude,  terrifying  motions,  with 
special  appendages  and  mimicry.  Two  other  caterpillars  in 

"terrifying  attitudes" 
are  shown  on  Plate 
78,  and  in  Fig.  41 
is  shown  a  moth  in 
what  is  said  to  be  its 
"  terrifying  attitude." 
Another  reputed 
instance  of  mimicry 
sometimes  mentioned 
is  that  of  the  marking 
on  the  tips  of  the  wings  of  some  of  the  large  moths,  which 
very  closely  resembles  the  head  of  a  cobra  with  its  expanded 
hood,  even  the  spectacle-like  marks  on  the  back  of  the  hood 
being  reproduced  (Fig.  42).  I  know,  however,  of  no  experi- 
ments which  test  the  effect  of  this  appearance  upon  insect- 
eating  animals,  and  without  such  experiments  we  have  no 
right  to  regard  the  fancied  resemblance  as  significant. 

There  are  examples  of  mimicry  among  the  vertebrates. 
Several  venomous  species  of  Elaps,  the  coral  snake,  are 
conspicuously  banded  with  red  and  black,  or  with  red  and 
black  and  yellow,  and  these  venomous  species  are  each 
imitated  by  other  species  of  harmless  snakes,  belonging  to 


FIG.  41.  —  "Terrifying  attitude"  of  a  moth  (Smerinthus 
ocellata).     [After  WEISMANN.] 


ORGANIC  EVOLUTION 


different  genera  (Plate  79).  Many  of  our  common  Ameri- 
can snakes  imitate  poisonous  serpents  in  one  peculiar  habit, 
though  not  in  exact  color.  Poisonous  serpents  when  cor- 
nered and  irritated  have  the  habit  of  flattening  their  heads 


FlG.  42.  —  Moth  from  India  (Attacus  atlas),  at  the  tips  of  whose  wings  are  markings  resembling 
•  those  upon  the  head  of  a  cobra. 


so  that  they  become  even  more  triangular  than  when  at  rest, 
and  they  show  a  pugnacity  that  is  very  forbidding.  Most 
of  our  little  harmless  snakes,  when  cornered,  will  behave  in 
much  the  same  manner,  flattening  the  head  and  making  it 
triangular,  and  by  their  hissing  and  striking  they  seem  to 
suggest  that  they  are  dangerous. 


'1  lie  snakes  in  group  A  have  poisonous  langs;    those  in 


PLATE  79  —  Mimicry  in  snakes. 
group  B  are  harmless. 

a.  Elaps  dumerili.  New  Granada.  6.  Elaps  lemniscatus,  Brazil,  c.  Elaps  semipartitits,  New 
Granada,  ti.  Elaps  psyche,  Brazil,  e.  Elaps  corallimus,  Brazil,  Central  America.  /.  Ophibolus 
doliatus,  Southern  North  America  and  Central  America.  g.  Pliocercus  elapsides,  Mexico. 
h.  Oxyrrhopus  trigemimis,  Brazil.  *'.  Pliocercus  euryzonus,  New  Granada.  /.  Erythrolamprus 
esculapii,  Brazil.  k.  Cemophora  coccinea.  Southern  United  States.  /.  Erythrolamprus  venus- 
tissimi/s,  Brazil,  Central  America.  [After  COPE.] 


COLOR  IN  ANIMALS  147 

There  are  a  few  examples  of  mimicry  among  birds.  Let 
me  quote  from  Wallace's  Darwinism  a  description  of  prob- 
ably the  best  example.  "  More  perfect  cases  of  mimicry 
occur  between  some  of  the  dull-colored  orioles  in  the 
Malay  Archipelago  and  a  genus  of  large  honey-suckers,  the 
Tropidorhynchi  or  'friar-birds'  (Plate  80).  These  latter  are 
powerful  and  noisy  birds  which  go  in  small  flocks.  They 
have  long,  curved,  and  sharp  beaks,  and  powerful,  grasping 
claws;  and  they  are  quite  able  to  defend  themselves,  often 
driving  away  crows  and  hawks  which  venture  to  approach 
them  too  nearly.  The  orioles,  on  the  other  hand,  are  weak 
and  timid  birds,  and  trust  to  concealment  and  to  their  retir- 
ing habits  to  escape  persecution.  In  each  of  the  great 
islands  of  the  Austro- Malayan  region  there  is  a  distinct 
species  of  Tropidorhynchus,  and  there  is  always  along  with 
it  an  oriole  that  exactly  mimics  it.  All  the  Tropidorhynchi 
have  a  patch  of  bare  black  skin  around  the  eyes,  and  a  ruff 
of  curious,  paler,  recurved  feathers  on  the  nape,  whence  their 
name  of  friar-birds,  the  ruff  being  supposed  to  resemble  the 
cowl  of  a  friar.  These  peculiarities  are  imitated  in  the 
orioles  by  patches  of  feathers  of  corresponding  colors ; 
while  the  different  tints  of  the  two  species  in  each  island  are 
exactly  the  same.  Thus  in  Bourru  both  are  earthy  brown ; 
in  Ceram  they  are  both  washed  with  yellow  ochre ;  in  Timor 
the  under  surface  is  pale  and  the  throat  nearly  white,  and 
Mr.  H.  O.  Forbes  has  recently  discovered  another  pair  in 
the  island  of  Timor  Laut.  The  close  resemblance  of  these 
several  pairs  of  birds,  of  widely  different  families,  is  quite 
comparable  with  that  of  many  of  the  insects  already 
described.  It  is  so  close  that  the  preserved  specimens  have 
even  deceived  naturalists,  for,  in  the  great  French  work. 


I48  ORGANIC  EVOLUTION 

Voyage  de  r Astrolabe,  the  oriole  of  Bourru  is  actually 
described  as  a  honey-sucker,  and  Mr.  Forbes  tells  us  that, 
when  his  birds  were  submitted  to  Dr.  Sclater  for  description, 
the  orioles  and  the  honey-suckers  were,  previous  to  close 
examination,  considered  to  be  the  same  species." 

Well-authenticated  examples  of  mimicry  among  mam- 
mals, or  other  vertebrates  than  the  birds  and  reptiles,  are 
not  numerous.  Among  the  invertebrates,  outside  the  classes 
of  the  insects  and  the  spiders,  there  are  some  instances 
known,  but  as  they  are  not  very  frequent,  and,  as  they  are 
seen  in  forms  which  are  less  generally  known,  we  will  not 
refer  to  them. 

Wallace  mentions  five  conditions  which  are  always  ful- 
filled in  cases  of  mimicry.  Let  me  quote  his  statement 
of  these. 

"  i.  The   imitative   species   must   occur   in    the    same    area 
and  occupy  the  very  same  station    as    the   imitated. 
"  2.  The  imitators  are  always  the  more  defenceless. 
"  3.  The  imitators  are  always  less  numerous  in  individuals. 
"  4.  The  imitators  differ  from  the  bulk  of  their  allies. 
"  5.  The  imitation,  however  minute,  is    external  and  visible 
only,  never  extending   to    internal    characters    or   to 
such  as  do  not  affect  the  external  appearance." 

The  instances  thus  far  mentioned  are  all  of  protective 
mimicry.  Of  aggressive  mimicry  there  are  but  very  few 
instances  known.  Some  of  the  hunting  spiders  are  very 
like  the  flies  on  which  they  prey;  possibly  also  the  ant- 
like  spiders  can  more  readily  approach  their  prey  because 
of  their  resemblance  to  ants  which  may  not  be  so  much 
avoided  by  small  flies  (Figs.  38  and  39).  Certain  insects, 


- 


COLOR   IN  ANIMALS 


149 


whose  larvae  are  parasitic  upon  other  insects,  closely 
resemble  the  form  upon  which  their  larvae  are  parasitic, 
being  enabled  thus  to  escape  detection  when  approaching 
to  lay  their  eggs  in  the  nests  of  the  species  whose  members 
will  become  infested  with  their  larvae.  These  parasites 
live  chiefly  with  or  upon  different  kinds  of  bees  and  ants. 

Signals  and  recognition  marks. 

Signals  and  recognition  marks  are  seen  in  many  animals. 
Birds  and  mammals  especially  display  these.  Our  com- 
mon rabbit,  when  startled, 
lifts  his  tail  as  he  runs, 
the  white  on  the  under 
surface  and  on  the  flanks 
under  the  tail  showing 
almost  like  a  flash  of 
white  light.  This  brill- 
iant white  patch  is  sup- 
posed to  serve  as  a  signal 
to  other  rabbits,  especially 
the  young,  to  seek  in 
flight  safety  from  some 
impending  danger  (Fig.  43).  Our  common  eastern  deer 
have  a  similar  white  spot  on  the  under  surface  and  below 
the  tail,  which  serves  the  same  purpose.  Some  of  the 
western  American  antelopes  have  upon  the  flanks  a  much 
larger  patch  of  long  white  hairs,  which  when  expanded  by  the 
contraction  of  the  skin  muscles  and  the  consequent  erec- 
tion of  the  hairs,  flashes  out  as  a  white  signal  visible  on 
the  plains  for  miles  (Plate  81).  Similar  white  rump  patches 
are  found  in  some  of  the  African  gazelles. 


FIG.  43.  —  Common  "cottontail"  rabbit,  which  is 
startled  and  about  to  run.  The  tail  is  lifted  enough 
to  show  a  part  of  its  white  under  surface  and  the  white 
rump  patch.  —  From  a  photograph  from  life  by  E.  R. 
Wan  en. 


!50  ORGANIC  EVOLUTION 

Wallace  interprets  some  of  the  very  distinct  marks  on 
different  birds,  such  as  the  white  outer  tail  feathers  which 
show  in  flight,  and  the  streaks  and  spots  about  the  head 
and  neck,  as  recognition  marks,  by  which  the  individuals  of 
the  same  species  recognize  each  other,  often  at  consider- 
able distances.  Such  marks  are  seen  in  our  common  kill- 
deer  and  in  the  night-hawk  (Plate  82).  Probably  this  is  a 
true  explanation  of  one  use  of  such  marks. 

Confusing  coloration. 

Dr.  C.  Hart  Merriam  has  suggested  another  use  for 
certain  color  markings  that  have  sometimes  been  described 
as  signals  or  recognition  marks.  All  must  have  noticed 
that  many  of  the  butterflies  have  the  upper  surface  of 
the  wings  brightly  colored,  while  the  under  surface  is  dull, 
and  that  these  forms,  when  at  rest,  close  the  wings,  dis- 
playing the  protectively  colored  under  surface.  This  is 
markedly  true  of  the  beautiful  leaf-butterfly,  Kallima  inachis, 
(Plate  83,  A  and  B}.  These  insects  are  very  noticeable 
when  in  flight,  but  when  they  light  and  close  the  wings, 
their  sudden  disappearance  is  most  startling  and  confusing, 
greatly  increasing  the  difficulty  of  observing  their  resting- 
place.  Many  of  the  moths,  which,  when  at  rest,  hold  the 
posterior  wings  covered  by  the  front  wings,  show  a  very 
similar  condition,  the  back  wings  being  brilliantly  colored 
above,  while  the  front  wings  are  dull  (Plate  83,  G,  H,  and  K). 
These  moths  do  not  fly  by  day,  unless  disturbed,  and  will  be 
well  protected  by  their  dull  color.  In  flight,  however,  the 
bright  color  of  their  posterior  wings  is  very  noticeable  and 
serves  to  make  their  disappearance  more  disconcerting  when 
they  alight.  The  yellow  or  red  under-wings  of  grasshoppers 


PLATE 


i.  —  Antelope  showing  danger  signal.  —  From  Wallihan's  Camera  S'tots  at  Big  Game,  by  permis- 
sion of  Mr.  Wallihan  and  of  Doubleday,  Page  and  Co. 


PLATE  82.  —  A.  Killdeer,  or  ring-marked  plover  (/Egialith  vocifera).  —  From  an  exhibit  in 
the  United  States  National  Museum.  B.  Nighthawk  (Chordeiles  virginianus)  spread  out  on  a 
log  in  such  a  way  as  to  show  the  white  marks  on  the  wings  and  tail. 


PLATE  83.  —  CONFUSING  COLORATION. 


A  and  B.  Kallima  inachis.  Cand  D.  Grapta  sp.  E  and  F.  Hebomoia  glaucippe.  G  and 
H.  Catocala  concumbens.  I  and  J.  Junonia  sp.  K.  Phyllodes  verhuellis.  L.  Dissosteira  Caro- 
lina. M.  Hibiscus  tuber culatus. 


COLOR  IN  ANIMALS  151 

and  their  noisy,  jerky  flight  render  them  very  conspicuous 
when  on  the  wing  (Plate  83,  L  and  M);  This  makes  their 
sudden  disappearance  upon  alighting  all  the  more  startling 
and  confusing.  Any  one  who  has  attempted  to  catch  the 
common  brown,  roadside  grasshoppers  will,  I  am  sure,  ac- 
cept this  explanation  of  one  use  of  the  conspicuous  color 
of  their  hind  wings.  When  at  rest  they  can  be  seen  only 
by  the  keenest  attention  and  closest  observation,  but  when 
in  motion  they  are  seen  by  the  most  careless  observer.  The 
sudden  mental  change  from  careless  observation  of  the. 
brilliant  color  and  noisy  flight  to  the  close  scrutiny  necessary 
to  detect  these  grasshoppers  when  quiet  is  very  difficult,  and 
is  a  change  one  does  not  succeed  in  making  without  much 
practice. 

Some  birds  which  are  in  general  inconspicuously  colored 
have  white  or  some  other  bright  color  upon  the  wing  or  tail 
feathers,  which  becomes  visible  in  flight.  Examples  are  the 
night-hawk,  the  Junco,  the  vesper-sparrow.  The  night-hawk 
is  so  colored  as  to  be  observed  only  with  great  difficulty 
when  at  rest  upon  a  log  or  upon  the  ground  (Plate  51,  A). 
It  often  lies  quiet,  trusting  to  its  inconspicuousness,  until  one 
nearly  steps  upon  it.  When  flushed,  however,  it  flies  away 
with  a  jerky,  zigzag  flight,  showing  in  the  most  conspicuous 
manner  its  clear  white  spots  upon  the  wings  and  tail  (Plate 
82,  £}.  The  great  contrast  between  its  conspicuousness  in 
flight  and  its  almost  invisible  character  when  at  rest  renders 
it  very  difficult  to  find  when  it  has  alighted. 

Merriam  would  give  a  similar  explanation  of  the  use  of 
the  conspicuous  bands  seen  upon  the  hips  and  tails  of 
the  desert  Kangaroo  rats  and  of  the  white  under  tail  of  the 
antelope,  squirrel,  cottontail  rabbit,  and  some  of  the  jack 


I52  ORGANIC  EVOLUTION 

rabbits,  all  of  which  markings  are  invisible  when  the  ani- 
mals crouch.  Some  of  the  desert  lizards  are  conspicuously 
banded  on  the  under  surface  of  the  tail,  which  they  elevate 
and  arch  over  the  back  when  startled,  running  with  great 
rapidity  for  a  short  distance,  then  suddenly  crouching,  until 
only  the  protectively  colored  back  is  visible,  or  rather  in- 
visible, among  the  rocks  and  sand. 

These  animals,  which  show  such  confusing  coloration, 
generally  run  or  fly  in  an  irregular  course,  and  just  before 
they  come  to  rest  they  cover  the  conspicuous  color  and  fre- 
quently dodge  to  one  side,  so  that  they  lie  unnoticed  at  some 
distance  to  one  side  of  the  spot  where  they  were  last  seen 
by  the  observer. 

It  is  of  course  possible  that  in  many  cases  the  same 
markings  may  serve  the  double  purpose  of  recognition  marks 
or  signals  and  of  increasing  the  startling  effect  of  the  sudden 
disappearance  of  their  possessors  when  they  come  to  rest. 

Sexual  coloration. 

We  have  already  had  occasion,  in  connection  with  the 
discussion  of  sexual  selection,  to  refer  to  the  differences  in 
the  appearance  of  the  males  and  the  females  of  many  species. 
These  differences  are  often  largely  differences  in  color,  and 
should  be  mentioned  in  any  treatment  of  the  phenomena  of 
color  in  animals.  The  use  of  these  sexual  colors  is, 
apparently,  to  render  the  male  attractive  to  the  female  and 
secure  her  as  his  mate.  In  our  discussion  of  sexual  selection 
we  said  that  these  brilliant  colors  of  the  male  are  seen 
among  birds,  lizards,  fishes,  spiders,  in  many  species  of 
butterflies,  and  in  some  insects.  We  will  stop  here  to  men- 
tion but  a  few  instances  from  these  groups.  Among  birds 


PLATE  84.  —SEXUAL  COLORATION  AND  MIMICRY  IN  BUTTERFLIES  AND  MOTHS. 

A.  Ornithoptera  priamus,  female.  B.  Ornithoptera  priamus,  male.  C.  Calamesia  midama. 
male,  imitates  E.  D.  Calamesia  midama,  female,  imitates  /•'.  E.  Eupliea  midamus  male 
F.  Euplasa  midamus,  female. 


PLATE  85.— SEXUAL  COLORATION  AND  PROTECTIVE  COLORATION  IN  SPIDERS. 
A.  ffabrocestnm  splendens,   male.     B.   ffabrocestnm   splendens,    female.      C.   Phidippus  cardinal™, 
male.     D.   Tetragnatlia  laboriosa.     \A,  B  and  C  after  G.  \V.  and  E.  G.  PECKHAM.     D.  From  specimens 
given  by  II.  W.  I',RITCHER.| 


COLOR  IN  ANIM4LS  153 

think  of  the  brilliant  colors  of  the  male  and  the  more  modest 
coloring  of  the  female  in  the  peacock,  the  common  chickens 
(Plates  12-15),  the  argus  pheasant  (Plate  24,  A\  the  birds  of 
paradise,  the  oriole,  cardinal,  and  bobolink  (Plate  22),  the 
bluebird,  American  goldfinch,  and  the  indigo-bird.  Even  the 
robin  and  the  common  grackle,  or  blackbird,  show  brighter 
colors  in  the  male  than  in  the  female.  The  humming-birds 
also  are  good  illustrations  (Plate  26). 

The  brilliant  bronze-green-and-blue  neck  of  the  males  of 
our  common  eastern  tree  lizards  is  an  instance  of  sexual 
coloration.  Other  finer  examples  could  be  mentioned 
among  tropical  lizards.  In  many  species  of  fish  the  males 
are  much  brighter  colored  than  the  females,  and  they 
display  the  brilliant  colored  parts  of  the  body  to  full 
advantage  when  approaching  the  females  in  the  breeding 
season. 

Greater  brilliancy  of  color  in  the  male  than  in  the  female 
is  a  quite  general  rule  among  fishes,  and  it  is  important  to 
note  that,  in  those  cases  in  which  the  courting  habits  of 
species  with  bright-colored  males  have  been  observed,  the 
male  has  the  habit  of  displaying  to  the  greatest  advantage 
his  bright  colors  when  he  approaches  the  female. 

Not  only  do  we  find  differences  in  color  between  the 
sexes  among  the  fishes ;  we  also  find  instances  of  differ- 
ences in  form,  the  males  having  certain  ornamental  append- 
ages not  seen  in  the  females,  or  the  fins  of  the  males  being 
larger  (Plate  32). 

See  also  Plates  84  and  85  for  illustrations  of  differences 
in  coloration  between  the  sexes  in  butterflies,  moths,  and 
spiders. 


!54  ORGANIC  EVOLUTION 

Summary 

In  recapitulation,  then,  we  may  say  that,  aside  from  their  direct  physio- 
logical value,  many  colors  of  animals  are  useful  to  their  possessors  in  relation 
to  their  environment  or  to  their  special  life  habits.  Such  colors  we  may 
class  as  — 

Protective,  causing  their  possessors  to  harmonize  in  color  with  their  envi- 
ronment, and  so  escape  their  enemies  ; 

Aggressive,  rendering  their  possessors  inconspicuous,  and  so  enabling 
them  to  capture  their  prey; 

Alluring,  serving  to  attract  the  prey  of  the  forms  which  show  the  alluring 
coloration ; 

Warning  coloration,  conspicuous,  and  rendering  dangerous,  noxious,  or 
ill-flavored  species  readily  recognizable,  thus  saving  them  from  attack  : 

Immunity  coloration,  which  may  often  be  conspicuous,  because  its  pos- 
sessor being  protected  from  predacious  enemies  by  inaccessible  habitat,  or 
dangerous  or  disagreeable  character,  has  no  need  of  protective  coloration. 

Mimetic  coloration,  by  which  an  edible  species  is  protected  from  its 
enemies  by  its  resemblance  to  a  dangerous,  noxious,  or  ill- flavored  species  (pro- 
tective mimicry)  ;  or  by  which  a  species  is  brought  to  resemble  its  habitual 
prey  or  some  species  of  which  its  prey  is  not  afraid  (aggressive  mimicry)  ; 

Signals  and  recognition  marks,  by  which  individuals  of  a  species  may 
recognize  their  fellows  or  may  warn  them  of  impending  danger ; 

Confusing  coloration,  which  disconcerts  an  enemy  by  the  startling  differ- 
ence between  the  conspicuousness  of  the  individual  when  in  motion  and  its 
inconspicuous  character  when  at  rest ;  and 

Sexual  coloration. 

Color  in  plants. 

The  color  phenomena  in  plants  are  as  interesting  as 
those  in  animals,  and  are  as  intimately  connected  with 
the  theory  of  evolution.  They  are,  however,  not  so  well 
understood  in  some  of  their  aspects.  We  will  consider 
the  colors  of  plants,  chiefly  of  plant  blossoms,  only  as 
related  to  insects.  It  seems  to  be  wholly  probable  that 
the  colors  of  blossoms  have  been  developed  in  connection 
with  insects.  The  bright  colors  serve  to  attract  insects 
and  the  insect  visits  are  an  advantage  to  the  plants. 


— p 


PLATE  86.  —  Diagrams  of  various  flowers  to  show  the  arrangement  of  their  parts.  [After 
KERNER.] 

A.  Flower  of  Butomus  umbettatus,  in  which  all  the  parts  are  distinct.  P.  Flower  of  Phytohicca 
decandra,  in  which  the  five  carpels  are  somewhat  united  to  one  another.  C.  Flower  of  Gagea 
lutea,  in  which  the  three  carpels  are  united  to  form  a  single  pistil  with  one  style  hut  a  three-parted 
stigma. 

a,  anther;  c,  carpel  (the  carpels  when  fused  to  form  a  single  structure  are  called  a  pistil); 
p,  petal  (the  petals  taken  together  compose  the  corolla)  ;  ps,  pistil ;  s,  sepal  (the  sepals  as  a 
whole  compose  the  calyx)  ;  s/,  stamen,  at  the  tip  of  which  is  the  anther,  which  bears  the  pollen ; 
stg,  stigma,  the  tip  of  the  pistil  (it  is  adapted  to  receiving  the  pollen  in  fertilization). 


COLOR  JN  PLANTS 


155 


All  are  familiar  with  the  general  structure  of  plant 
blossoms  (Plate  86).  Within  the  brightly  colored  floral 
leaves  are  found  two  sets  of  reproductive  organs:  an  inner 
set,  female,  called  carpels,  or  when  united  as  they  com- 


FlG.  44.  —  Fertilization  in  the  rock-rose  ( Helianthemum  marifolium) .       [After  KERNKK  ] 

i.  A  single  flower,  natural  size.  2.  A  flower,  stripped  of  its  sepals  and  petals,  showing  the 
pistil  in  longitudinal  section.  The  pollen  grains  are  seen  upon  the  stigma,  and  their  tubes  are 
seen  passing  down  the  stalk  of  the  pistil  to  reach  the  ovules.  The  tubes  are  indicated  erroneously 
as  going  direct  to  the  openings  at  the  tips  of  the  ovules ;  actually  they  follow  a  more  devious 
course,  first  down  the  inside  wall  of  the  chamber  of  the  pistil  and  then  up  to  reach  the  apertures 
in  the  ovules ;  ov.  =  ovule,  stg.  —  stigma.  3.  A  more  enlarged  drawing  of  the  tip  of  the  pistil,  show- 
ing the  pollen  grains  and  the  sprouting  pollen  tubes.  4.  A  dry  pollen  grain.  5.  A  moistened 
pollen  grain  developing  its  tube.  6.  An  ovule,  showing  the  opening  at  its  tip  through  which  the 
pollen  tube  enters  to  effect  fertilization. 

monly  are,  together  composing  the  pistil ;  and  an  outer 
whorl  of  stamens,  or  male  organs.  The  ovules,  or  imma- 
ture seeds,  are  formed  within  the  pistil  (Fig.  44,  ov\  while 
the  pollen,  by  which  the  ovules  are  to  be  fertilized,  is 
formed  in  the  anthers  at  the  tips  of  the  stamens.  To 


I56  ORGANIC  EVOLUTION 

produce  a  seed  which  will  grow  and  give  rise  to  a  new 
plant,  pollen  from  a  stamen  must  be  deposited  on  the 
stigma,  or  tip  of  the  pistil;  here  it  will  sprout  and  send 
down  a  tube  within  the  pistil  to  reach  and  fertilize  an 
ovule  (Fig.  44,  2,  j>,  and  5),  which  then  becomes  a  seed 
capable  of  producing  a  new  plant.  Now,  it  has  been 
observed  over  and  over  again  that  if  a  pistil  is  impreg- 
nated with  pollen  from  another  plant  the  new  plants  com- 
ing from  the  seeds  thus  fertilized  will  often  be  stronger 
and  more  vigorous  than  if  they  had  been  developed  from 
seeds  fertilized  by  pollen  from  the  same  plant  that  formed 
the  seeds.  Cross-fertilization,  as  it  is  called,  is  advanta- 
geous. Self-fertilization  does  occur,  but  it  is  important  for 
most  species  that  cross-fertilization  should  come  in  every 
few  generations  at  least. 

Different  methods  of  fertilization  are  adopted  by  differ- 
ent kinds  of  plants.  The  flowerless  plants  have  their  own 
methods,  and  the  flowering  plants  usually  different  ones. 
We  are  here  interested  only  in  the  means  of  securing  fer- 
tilization adopted  by  the  flowering  plants.  Some  of  these, 
like  the  pines  and  other  evergreen  trees,  have  an  enormous 
amount  of  pollen  which  is  cast  out  into  the  air  in  great 
clouds  and  is  carried  by  the  winds  to  the  female  cones, 
there  to  fertilize  the  ovules  (Plate  87,  A\  There  are  many 
such  wind-fertilized  plants,  the  palms  and  grasses,  as  well 
as  the  cone-bearing  trees,  being  familiar  examples.  These 
do  not  use  insects  to  aid  in  carrying  pollen  to  fertilize 
their  ovules,  and  so,  as  every  one  knows,  they  have  no 
brilliantly  colored  blossoms  (Plate  87,  B\ 

By  far  the  larger  number,  however,  of  our  common 
flowering  plants  are  aided  in  securing  fertilization  by  the 


«  a 

Cfl 


COLOR  IN  PLANTS 


157 


insects  which  visit  their  blossoms.  The  petals  of  the 
flowers  usually  secrete  nectar,  which  is  attractive  to  insects, 
and  many  blossoms  have  an  odor  which  also  serves  to 
attract  insects.  The  nectar  is  a  sweet  fluid  secreted  by 
small  glands,  or  nectaries,  on  the  bases  of  the  petals.  It 
is  this  nectar  which  bees  gather  and  make  into  honey. 
The  odors  of  blossoms  are  caused  by  the  presence  of 
volatile  oils  usually  also  secreted  by  the  petals.  These 

odors  may  be 
such  as  are  agree- 
able to  our  nos- 
trils, as  are  the 
odors  of  the  rose, 
the  sweet  violet, 
the  trailing  arbu- 
tus, or  they  may 
be  to  us  disa- 
greeable, like  the 
odors  of  the  car- 
rion-flower and 
skunk-cabbage ; 
but,  whether  agreeable  to  us  or  not,  they  serve  to  secure  the 
visits  of  insects,  and  it  is  apparently  because  of  this  attrac- 
tiveness to  insects,  and  the  advantage  of  cross-fertilization 
in  which  the  insects  aid,  that  these  odors  and  the  nectar 
have  been  developed.  Many  insects  also  seek  the  pollen 
in  the  blossoms,  using  it  as  food,  and  most  plants  form  more 
pollen  than  is  needed  to  fertilize  their  ovules,  thus  having 
a  surplus  supply  upon  which  insects  may  draw  without  much 
or  any  injury  to  the  plant. 

The  insects  which  come  to  the  blossoms  to  gather  pollen 


FlG.  45. —  A  bee,  showing  the  hairs  on  the  head,  body,  and  legs 
Pollen  grains  are  shown  caught  in  the  hairs  on  the  legs. 


I58  ORGANIC  EVOLUTION 

or  nectar,  as  they  go  from  plant  to  plant,  will  carry  with 
them  pollen  dust  clinging  to  their  heads  and  legs  and  bodies 
(Fig.  45),  and  by  means  of  the  pollen  thus  carried  the  later 
plants  visited  will  secure  cross-fertilization.  One  might  per- 
haps think  that  the  insect  visitor  would  scatter  pollen  from 
one  plant  on  the  pistils  of  the  same  plant,  and  thus  cause 
self-fertilization  as  a  general  rule,  but  there  are  three  chief 
ways  in  which  this  is  commonly  prevented. 

Frequently  the  pollen  and  the  ovules  of  a  single  plant 
do  not  mature  at  the  same  time,  so  that  self-fertilization 
is  prevented. 

Many  plants  have  the  parts  of  their  blossoms  so  ar- 
ranged that  the  visiting  insect  will  go  to  the  nectar  first, 
without  coming  into  contact  with  the  pollen  until  he  is 
about  to  depart,  when  he  will  become  dusted  with  the 
pollen  and  carry  it  away  on  his  visit  to  the  next  blossom. 
Here,  on  the  way  to  the  nectar,  he  will  brush  against  the 
tip  of  the  pistil  and  give  to  it  some  of  the  pollen  he  has 
brought  from  the  first  plant,  thus  providing  a  means  of 
cross-fertilization.  Blossoms  are  often  remarkably  modified 
in  form  and  structure  to  prevent  in  this  way  self-fertiliza- 
tion. In  a  moment  we  will  consider  a  few  instances  of 
such  modification. 

The  third  and  almost  universal  method  of  preventing 
self-fertilization  is  a  physiological  one,  the  pollen  from  any 
given  plant  being  considerably  slower  to  sprout  on  a  pistil 
of  the  same  plant  than  it  is  upon  the  pistil  of  another 
plant;  thus,  even  though  the  pistil  of  any  blossom  be 
dusted  first  with  pollen  from  the  same  plant,  if,  later,  pollen 
from  another  plant  be  brought  to  the  blossom,  the  later- 
received  pollen  is  likely  to  be  that  which  will  effect  fertili- 


COLOR  IN  PLANTS  ^9 

zation,  because  of  its  more  prompt  sprouting  and  the  more 
rapid  growth  of  its  pollen  tube. 

Let  us  now  observe  a  few  illustrations  of  special  adap- 
tions in  the  form  of  blossoms,  by  which  plants  secure  cross- 
fertilization. 

The  flowers  of  Mitchella,  the  beautiful  little  partridge 
berry  of  our  woods,  are  adapted  to  secure  cross-fertilization 
by  insects  through  having  their  stamens  and  pistils  of  dif- 
ferent lengths  (Plate  88).  In  all  the  blossoms  of  one  plant 
the  stamens  will  be  long  and  the  pistils  short,  while  in  all  the 
blossoms  of  another  plant  these  relations  will  be  reversed,  the 
pistils  being  long  and  the  stamens  short.  An  insect  visiting 
these  blossoms  will  have  one  part  of  its  body  dusted  with 
pollen  from  the  short  stamens  and  another  part  with  pollen 
from  the  long  stamens.  As  it  passes  from  blossom  to  blos- 
som it  will  carry  pollen  from  the  short  stamens  of  one  flower 
to  the  short  pistils  of  other  flowers,  and  the  pollen  from  the 
long  stamens  will  be  carried  to  the  long  pistils.  In  this  way 
cross-fertilization  will  be  secured,  since  long  stamens  and 
long  pistils  do  not  occur  on  the  same  plant,  nor  are  short 
stamens  and  pistils  found  on  the  same  plant 

Many  orchids  show  an  interesting  method  of  using  insects 
to  secure  cross-fertilization.  In  these  species  the  stigma  is 
in  the  centre  of  the  flower,  while  the  anther  with  its  two 
pollen  masses  lies  above  the  stigma  (Plate  89,  A,  2  and  j). 
The  two  pollen  masses  protrude  a  little,  and  at  their  protu- 
berant ends  are  attached  to  a  sticky  "rostellum"  (Plate  89, 
A,  4).  The  corolla  of  the  flower  is  so  developed  as  to  form 
a  flat  landing-stage  for  the  visiting  bee  or  other  insect  (com- 
pare Plate  90,  C),  and  the  rostellum  protrudes  into  the 
centre  of  the  flower,  above  this  landing-place,  in  such  a  way 


!60  ORGANIC  EVOLUTION 

that  the  bee  in  entering  the  flower  to  reach  the  nectar 
will  press  its  head  against  the  rostellum  (5).  When  the 
bee  withdraws,  the  rostellum,  with  its  two  pollen  masses, 
sticks  to  its  forehead  (6),  and  the  pollen  masses  are  thus 
carried  to  the  next  blossom  visited.  At  first  the  pollen 
masses  stand  erect  upon  the  forehead  of  the  bee  (5  and  6), 
but,  as  the  bee  flies  through  the  air,  the  stalks  of  the  pollen 
masses  dry  slightly  and  bend  downward  (7),  so  that,  when 
the  bee  enters  another  flower,  the  pollen  masses  are  pressed 
against  the  stigma.  Thus  cross-fertilization  must  be  fre- 
quently effected,  the  bees  carrying  the  pollen  not  only  from 
blossom  to  blossom  of  the  same  plant,  but  also  from  one 
plant  to  another. 

The  flowers  of  Salvia  have  adopted  another  and  equally 
interesting  method  of  reaching  the  same  result.  In  these 
blossoms  the  stamens  are  hinged,  and  the  lower  end  of  the 
stamen,  below  the  hinge,  is  so  placed  that  a  bee,  in  entering 
the  blossom,  will  push  against  it,  and  in  doing  so  will  cause 
the  other  end  of  the  stamen  with  its  pollen  to  drop  down  and 
dust  the  back  of  the  bee  with  pollen  (Plate  89,  B,  /,  j,  ^,  and 
5).  This  pollen  will  be  carried  to  the  next  flower  visited  by 
the  bee.  Frequent  cross-fertilization  is  secured  by  another 
simple  character  of  these  blossoms.  The  pollen  is  mature  by 
the  time  the  blossom  bud  opens,  but  at  this  time  the  pistil  is 
short  and  lies  arched  in  the  upper  part  of  the  blossom  (/). 
As  the  flower  grows  older,  the  pistil  elongates  and  bends 
downward,  so  as  now  to  come  into  contact  with  any  insect 
which  may  visit  the  flower  with  its  load  of  pollen,  thus  secur- 
ing fertilization  (2).  As  the  pollen  and  the  pistil  are  not 
ready  for  fertilization  at  the  same  time,  no  blossom  will  be 
.self-fertilized ;  and,  as  the  insects  pass  frequently  from  plant 


PLATE  88.  — PARTRIDGE-BERRY  (Mitchella  repens). 


A.  Plant  with  blossoms  and  fruit.  —  From  Goodale's  Wild  Flowers  of  America,  by  the  courtesy 
of  Bradlee  Widden.  B.  Blossoms  with  long  pistil  and  short  stamens.  C.  Blossoms  with  long 
stamens  and  short  pistil.  —  B  and  C  from  Bastin's  College  Botany,  by  the  courtesy  of  G.  P.  Engel- 
hard and  Co. 


^1  2-g. 
£h  e  2 


PLATE  89.  —  A.  The  fertilization  of  an  orchid  by  a  wasp.     [After  KERNER.] 

i.  Flowering  spike  of  the  broad-leaved  helleborine  (Epipactys  latifolia)  upon  which  a  wasp  is 
alighting.  2.  Flower  of  the  same  seen  from  the  front.  3.  Side  view  of  the  same  flower,  with  the 
half  of  the  perianth  toward  the  observer  cut  away.  4.  The  two  pollen  masses  joined  by  the  sticky 
rostellum.  5.  The  same  flower  being  visited  by  a  wasp  which  is  licking  honey  and  at  the  same 
time  detaching  with  its  forehead  the  tip  of  the  rostellum  together  with  the  pair  of  pollen  masses. 
6.  The  wasp  leaving  the  flower  with  the  pollen  masses  cemented  to  its  head;  the  pollen  stalks  are 
erect.  7.  The  wasp  visiting  another  flower  and  pressing  its  forehead  with  the  pollen  masses  (which 
in  the  meantime  have  bent  down)  against  the  stigma.  I,  natural  size;  the  other  figures,  X  2. 

B.  Fertilization  of  Salvia  by  a  bumblebee.     [After  KERNER.] 

i.  Part  of  an  inflorescence  of  Salvia  glutinosa;  the  right-hand  flower  is  being  visited  by  a 
bumblebee,  and  the  pollen-covered  anther  is  in  the  act  of  striking  the  insect's  back.  2.  Another 
part  of  the  same  inflorescence  with  three  open  flowers  in  different  stages  of  development ;  the  left- 
hand  flowers  are  slightly  more  mature  than  the  right-hand  flower;  one  of  the  flowers  is  being 
visited  by  a  bumblebee  which  carries  on  its  back  pollen  from  a  younger  flower  and  is  rubbing  it 
off  on  ihe  deflexed  stigma.  3.  A  single  stamen,  showing  the  hinge  (h) .  4.  A  vertical  section 
through  a  blossom ;  the  arrow  indicates  the  direction  through  which  bumblebees  advance 
toward  the  interior  erf  the  flower.  5.  A  similar  section,  showing  how  the  anther  is  bent  down- 
ward by  a  bumblebee  pushing  against  the  bottom  of  the  stamen. 


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COLOR  IN  PLANTS  161 

to  plant,  as  well  as  from  blossom  to  blossom  of  the  same 
plant,  cross-fertilization  between  plants  must  frequently 
result. 

Aristolochia  sipho  illustrates  another  method  of  secur- 
ing cross-fertilization  through  insects.  The  flowers  of  this 
species  are  in  the  form  of  a  bent  tube  with  a  flaring  end, 
something  like  a  trumpet  (Plate  90,  A).  Flies  enter  the 
opening  of  this  tube,  but  find  their  egress  prevented  by  a 
mass  of  hairs  (Plate  90,  £)  which  fills  the  tube,  pointing 
toward  its  base,  allowing  the  flies  to  enter  but  not  to 
depart.  The  stigma  of  these  blossoms  is  a  large  top-shaped 
structure,  nearly  filling  the  base  of  the  tube.  Behind  it, 
and  inaccessible  to  the  flies,  are  the  three  biscuit-shaped 
anthers  with  their  pollen.  The  swollen  stigma  shrinks  as 
the  flower  grows  older,  and  if  the  flies  which  have  entered 
have  brought  pollen  with  them  and  have  fertilized  the 
stigma  its  shrinking  is  hastened.  After  the  stigma  has 
shrivelled,  the  flies,  as  they  wander  about  their  prison,  can 
reach  the  pollen  and  will  become  well  dusted  with  it.  Now 
the  hairs  which  have  prevented  their  departure  dry  and 
shrivel  and  the  flies  are  set  free  to  seek  another  blossom 
and  fertilize  its  ovules. 

Each  of  these  general  methods  of  securing  cross-fertili- 
zation which  we  have  illustrated  is  used  by  a  considerable 
number  of  plants,  and  there  are  scores  of  other  devices  to 
which  we  have  not  space  to  refer.  Many  of  these  are  vividly 
described,  with  good  pictorial  illustrations,  in  Kerner's  The 
Natural  History  of  Plants,  the  English  translation  of  which, 
by  Oliver,  is  published  by  Henry  Holt  &  Company. 

Enough  has  been  said  to  emphasize  the  importance  to 
the  plant  of  insect  visits.  We  have  seen  that  by  the  secre- 


!62  ORGANIC  EVOLUTION 

tion  of  nectar  and  odoriferous  oils,  and  by  the  formation  of  a 
surplus  supply  of  pollen,  plants  invite  the  visits  of  insects, 
and  that  they  sometimes  adopt  remarkable  means  to  secure 
cross-fertilization  by  the  aid  of  the  visiting  insects.  Careful 
experiments  have  been  made  by  numerous  competent 
students  to  determine  if  color  in  itself  is  recognized  by 
insects  of  different  sorts.  These  have  established  the  fact 
that  color  is  recognized  by  insects  of  many  kinds,  and  that 
to  certain  species  of  insects  different  colors  have  different 
degrees  of  attraction.  Also  it  has  been  shown  that  the  most 
attractive  color  is  not  always  the  same  for  two  species  of 
insects. 

Lord  Avebury's  experiments  upon  bees  are  worth  our 
attention  for  a  moment,  as  an  illustration  of  the  methods 
which  have  enabled  us  to  draw  these  conclusions.  In  a 
very  brief  summary  of  an  extended  series  of  experiments 
Lord  Avebury  says :  "  I  placed  slips  of  glass  with  honey 
on  papers  of  various  colours,  accustoming  different  bees  to 
visit  special  colours,  and  when  they  made  a  few  visits  to 
honey  on  paper  of  a  particular  colour,  I  found  that  if  the 
papers  were  transposed  the  bees  followed  the  colour." 
Describing  another  series  of  experiments,  he  says :  "  I  took 
slips  of  glass  of  the  size  generally  used  for  the  microscope, 
viz.  three  inches  by  one,  and  pasted  them  on  slips  of  paper 
coloured  respectively  blue,  green,  orange,  red,  white,  and 
yellow.  I  then  put  them  on  a  lawn,  in  a  row,  about  a  foot 
apart,  and  on  each  put  a  second  slip  of  glass  with  a  drop  of 
honey.  I  also  put  with  them  a  slip  of  plain  glass  with  a 
similar  drop  of  honey.  I  had  previously  trained  a  marked 
bee  to  come  to  the  place  for  honey.  My  plan  then  was, 
when  the  bee  returned  and  had  sipped  for  about  a  quarter  of 


COLOR   IN  PLANTS  ^3 

a  minute,  to  remove  the  honey,  when  she  flew  to  another 
slip.  This  then  I  took  away,  when  she  went  to  a  third ;  and 
soon.  In  this  way  —  as  bees  generally  suck  for  three  or 
four  minutes  —  I  induced  her  to  visit  all  the  drops  succes- 
sively before  returning  to  the  nest.  When  she  had  gone  to 
the  nest  I  transposed  all  the  upper  glasses  with  the  honey, 
and  also  moved  the  coloured  glasses.  Thus,  as  the  drop  of 
honey  was  changed  each  time,  and  also  the  position  of  the 
coloured  glasses,  neither  of  these  could  influence  the  selec- 
tion by  the  bee. 

"  In  recording  the  results  I  marked  down  successively  the 
order  in  which  the  bee  went  to  the  different  coloured 
glasses.  For  instance,  in  the  first  journey  from  the  nest,  as 
recorded  below,  the  bee  lit  first  on  the  blue,  which  accord- 
ingly I  marked  i ;  when  disturbed  from  the  blue,  she  flew 
about  a  little  and  then  lit  on  the  white,  which  I  marked  2  ; 
when  the  white  was  removed,  she  settled  on  the  green,  which 
was  marked  3  ;  and  so  on  successively  on  the  orange,  yellow, 
plain,  and  red.  I  repeated  the  experiment  a  hundred  times, 
usine  two  different  hives  —  one  in  Kent  and  one  in  Middle- 

O 

sex  —  and  spreading  the  observations  over  some  time,  so  as 
to  experiment  with  different  bees  and  under  varied  circum- 
stances. Adding  the  numbers  together,  it  of  course  follows 
that  the  greater  the  preference  shown  for  each  colour  the 
lower  will  be  the  number  standing  against  it. 

"  The  following  table  gives  the  first  day's  observations  in 
extenso :  — 


1 64 


ORGANIC  EVOLUTION 


"JOURNEYS 

BLUE 

GREEN 

PLAIN  GLASS  ;    ORANGE 

RED 

WHITE 

YELLOW 

I 

I 

3 

6 

4 

7 

2 

5 

2 

5 

4 

7 

6 

I 

2 

3 

3 

I 

4 

7 

6 

5 

3 

2 

4 

2 

4 

6 

7 

5 

i 

3 

5 

I 

4 

7 

2 

6 

5 

3 

6 

I 

2 

3 

6 

5 

4 

7 

7 

2 

i 

4 

7 

3 

5 

6 

8 

3 

4 

6 

2 

7 

5 

i 

9 

5 

i 

7 

4 

6 

3 

2 

10 

i 

6 

7 

5 

3 

2 

4 

ii 

4 

6 

5 

2 

7 

3 

1 

26 

39 

65 

51 

55 

35 

37" 

The  order  of  preference  here  indicated  is,  we  see,  be- 
ginning with  the  most  favored,  blue,  white,  yellow,  green, 
orange,  red,  and  the  plain  glass.  A  much  larger  number 
of  experiments  by  the  same  method  gave  the  following 
figures:  blue  275,  white  349,  yellow  405,  red  413,  green 
427,  orange  440,  plain  glass  491.  We  may  say,  then,  that 
bees  show  a  strong  preference  for  blue,  that  they  like 
white  next,  and  that  yellow,  red,  green,  and  orange  are 
about  equally  attractive,  and  are  all  preferred  to  uncolored 
objects. 

Other  experiments  by  Lord  Avebury  show  that  wasps 
have  a  decided  color  sense  and  are  able  to  distinguish 
vermilion,  orange,  blue,  white,  yellow,  and  green,  but  that 
they  do  not  show  a  very  decided  color  preference.  Similar 
results  have  also  been  obtained  by  Dr.  and  Mrs.  Peckhairu 
Experiments  upon  most  other  insects  are  more  difficult 
to  perform,  for  they  do  not  have  nests  in  which  they  live 
together  and  to  which  they  return  after  each  hunting  trip, 
or  in  which  they  store  honey,  returning  time  after  time  to 


COLOR  IN  PLANTS  165 

the  flowers  for  nectar.  Most  insects  eat  their  fill  and 
then  fly  away  and  do  not  return.  It  is  possible,  though,  by 
observation  of  flowers  in  nature  to  determine  what  kinds 
of  insects  are  their  most  frequent  visitors.  In  this  manner 
we  can  determine  that  "  white  flowers  are  especially  visited 
by  small  flies;  that  flowers  which  depend  upon  beetles 
for  fertilization  are  frequently  yellow;  that  those  which 
especially  bid  for  the  favor  of  bees  and  butterflies,"  the 
nectar  gatherers  par  excellence,  "are  usually  red,  purple, 
lilac,  or  blue." 1 

Since  the  visits  of  insects  are  so  valuable  to  plants  in 
securing  cross-fertilization,  it  is  easy  to  see  that  natural 
selection  would  be  likely  to  bring  about  the  bright  colora- 
tion of  flowers ;  and,  as  insects  of  different  kinds  have 
different  color  preferences,  the  color  of  any  sort  of  flower 
is  likely  to  be  such  as  to  attract  the  kind  of  insect  best 
adapted  to  secure  its  cross-fertilization.  And,  in  general, 
we  may  say  that  the  observations  upon  the  colors  of  flowers 
agree  with  these  conclusions. 

The  most  assiduous  honey  gatherers  are  the  bees  and 
the  butterflies,  and  it  is  interesting  to  observe  that  the 
most  highly  specialized  flowers  in  the  different  families  of 
plants  are  usually  red  or  purple  or  blue,  being  thus  espe- 
cially attractive  to  these  insects  whose  preference  is  for 
these  same  colors. 

Much  has  been  written  about  other  principles  in  the 
coloration  of  blossoms,  their  original  color,  the  order  of 
development  of  the  several  colors,  the  way  in  which  new 
colors  arise,  the  parts  of  the  petals  upon  which  these  new 
colors  are  most  likely  to  appear,  the  meaning  of  variega- 

1  Grant  Allen,  The  Colours  of  Flowers. 


l66  ORGANIC  EVOLUTION 

tion  in  the  colors  of  petals,  the  colors  of  degenerate 
blossoms,  and  many  other  subjects  of  much  interest;  but, 
as  the  conclusions  to  be  drawn  from  the  great  number 
of  observations  are  still  very  much  in  dispute,  it  seems 
unwise  for  us  to  attempt  further  discussion  along  these 

lines.1 

There  is  one  further  thing  in  this  connection  to  which 
it  is  well  to  call  attention.  Many  highly  specialized  flowers 
have  developed  unusual  shapes  so  as  to  cause  the  visiting 
insects  to  enter  the  blossoms  by  the  path  most  likely  to  bring 
them  into  contact  with  the  pistil  and  the  pollen  in  such  a 
way  as  to  insure  cross-fertilization,  and  have  provided 
special  lighting  spots  or  platforms  for  their  visitors  (Plate 
90,  C;  compare  Plate  89),  and  these  are  often  spotted  and 
streaked  in  such  a  way  as  to  make  them  conspicuous.  More 
interesting  still  is  the  fact  that  these  streaks  are  usually  so 
arranged  as  to  point  the  way  to  the  nectaries,  guiding  the 
insect  along  the  right  path,  the  pistil  and  the  anthers  being 
so  placed  as  to  come  into  contact  with  the  body  of  the  insect 
in  the  most  advantageous  manner  as  it  passes  along  this 
prescribed  way. 

MAN    IN    RELATION   TO    EVOLUTION 

Naturally  the  subject  of  the  relation  of  humankind  to 
evolution  is  one  of  particular  interest  to  us.  Study  of 
human  anatomy  shows  mankind  to  be  probably  a  single 
species,  belonging  to  the  Primates,  a  group,  of  the  Mam- 
malia, including,  besides  man,  the  lemurs,  and  the  apes  and 

1  The  reader  will  find  Grant  Allen's  The  Colours  of  Flowers^  which  treats  of 
these  subjects,  a  most  interesting  and  suggestive  book,  but  hd  should  remember 
'that  Allen's  conclusions  are  not  accepted  by  botanists. 


EVOLUTION   OF  MAN  167 

monkeys  of  the  eastern  and  western  hemispheres.  Man  is 
most  nearly  related  to  the  Simiidcz,  the  tailless  apes  of 
Asia  and  Africa,  including  the  gibbon,  the  orang,  the 
chimpanzee,  and  the  gorilla.  It  is  usual  to  place  human- 
kind in  a  distinct  family  of  Primates,  Hominida.  It  is  now 
the  general  consensus  of  opinion  that  we  should  recognize 
but  a  single  species  and  distinguish  as  subspecies  the  sev- 
eral races  of  men. 

As  an  illustration  of  some  of  the  reasons  for  asserting 
that  men  are  primates  and  are  closely  related  to  the  Simi- 
idce,  glance  at  the  illustration  of  the  skeletons  of  representa- 
tives of  four  genera  of  Simiidce  and  of  man  (Plate  91,  A\ 
Part  for  part  the  skeletons  are  the  same  in  all  fundamental 
regards.  Look  at  but  a  single  group  of  bones,  those  com- 
posing the  pelvis  (Plate  91,  B\  The  larger  bones,  the 
sacrum,  and  the  coccyx  show  the  closest  resemblances  in 
man  to  what  we  see  in  the  gorilla.  The  relative  size  and 
shape  is  slightly  different,  and  man  has  lost  one  of  the 
coccygeal  bones  still  seen  in  the  gorilla,  but  in  all  essential 
features  the  two  sets  of  bones  are  closely  similar.  Similar 
comparisons  with  a  similar  result  might  be  made  'between 
the  hands,  feet,  sterna,  ribs,  spinal  columns,  teeth  (Plate  92, 
A\  bones  of  the  skull,  etc. 

But  let  us  turn  to  structures  other  than  the  skeleton. 
Passing  by  the  close  resemblance  between  the  vital  organs, 
the  muscles,  and  the  other  important  organs  (Plate  92,  B\ 
observe  again  some  of  the  remarkable  similarities  in  certain 
minor  details,  to  some  of  which  we  have  before  referred. 
We  think  of  the  hairiness  of  the  apes  as  distinguishing 
them  rather  sharply  from  man,  but  in  reality  the  whole  of 
the  human  body  is  covered  with  hair,  save  the  palms  of  the 


!68  ORGANIC  EVOLUTION 

hands,  the  soles  of  the  feet,  and  the  backs  of  the  terminal 
joints  of  the  fingers  and  toes  ;  and  these  same  portions  are 
naked  in  the  apes.  Not  only  does  hair  clothe  the  whole 
human  body,  the  slant  of  the  hair  in  the  several  regions  of 
the  body  is  the  same  that  we  observe  in  the  apes  (Plate  93). 
Therefore,  even  to  minute  details,  the  apes  and  man  can  be 
compared  as  to  the  presence  and  slope  of  hair;  the  only 
considerable  difference  in  the  condition  of  the  hair  in  the 
two  being  in  the  length  and  the  coarseness  of  the  indi- 
vidual hairs. 

Observe  another  minute  characteristic,  one  often  seen 
in  human  ears  (Plate  94).  In  many  monkeys  the  ears  are 
pointed  and  do  not  show  any  recurved  edge  such  as  is  seen 
in  the  ears  of  apes  and  men  (ear  of  Barbary  ape,  Plate  94). 
On  the  recurved  edge  of  the  human  ear  and  that  of  apes 
there  is  often  a  portion  slightly  more  developed  than  the 
rest,  showing  as  a  wider  place  (Plate  94),  or  even  a  point 
(Plate  95,  A  and  B]  on  the  reflected  edge.  This  corre- 
sponds to  the  point  seen  in  the  ears  of  the  lower  monkeys, 
only  in  their  ears  the  point  is  erect,  the  edge  of  the  ear  not 
being  folded  over. 

The  apes  and  man  have  the  tail  greatly  reduced,  it 
being  represented  merely  by  the  coccyx,  a  reminiscence  of 
the  ancestral  condition  when  functional  tails  were  present. 
It  is  interesting  to  know  that  there  have  been  instances  in 
which  a  human  being  has  retained  in  an  abnormally  highly 
developed  condition  the  muscles  which  represent  the  func- 
tional muscles  of  this  ancestral  tail  (Plate  95,  Q.  In  a 
similar  manner,  while  our  ears  are  slightly,  if  at  all,  movable, 
we  retain  in  a  vestigial  condition  the  muscles  which  in  some 
ancestor  must  have  served  to  move  the  ears  (Plate  96,  A). 


GiMn. 


PLATE  91.  —  A.  Skeletons  of  man  and  four  apes.  [After  HUXLEY.]  i.  Man.  2.  Gorilla. 
3.  Chimpanzee.  4.  Orang.  5.  Gibbon.  B.  Pelvis  of  man,  gorilla,  and  gibbon.  [After 
HUXLEY.] 


Chimpanzee. 


PLATE  92.  —  A.  Teeth  of  man  and  gorilla.     [After  HUXLEY.]    B.  Cerebral  hemispheres  of  man 
and  chimpanzee.     [After  HUXLEY.] 


PLATE  93.  —  Hair  tracts  on  the  arms  and  hands  of  a  man  and  a  male  chimpanzee.  Drawn 
from  life.  Observe  that  in  the  corresponding  regions  the  direction  of  the  slope  of  the  hairs  is  the 
same.  —  From  Romanes'  Darwin  and  After  Darwin,  by  the  courtesy  of  The  Open  Court  Publish- 
ing Company. 


PLATE  95.  —  A.  Head  of  foetus  of  an  orang-outang;  observe  the  pointed  ear.  [After  DAR- 
WIN.] B.  A  human  ear  in  which  a  point  is  present  upon  the  recurved  edge.  [After  DARWIN.] 
C.  Front  and  back  view  of  an  adult  human  sacrum,  showing  an  abnormal  persistence  of  vestigial 
tail  muscles.  —  From  Romanes'  Dai  win  and  After  Darwin,  by  the  courtesy  of  The  Open  Court 
Publishing  Company. 


MAN 


MAN 
FOETAL 


PI.ATF.  96.  —  A.  Muscles  of  the  human  ear.  —  From  Gray's  Anatomy.  B.  Vermiform  appen- 
dices of  oiang,  man,  and  human  fetus.  — From  Romanes'  Darwin  and  After  Darwin,  by  the 
courtesy  of  The  Open  Court  Publishing  Company. 


EVOLUTION   OF  MAN  169 

The  vermiform  appendix  is  less  developed  in  man  than 
in  the  apes,  and  in  an  adult  man  is  relatively  smaller  than 
in  the  human  foetus  (Plate  96,  B\ 

At  the  inner  angle  of  the  human  eye  is  a  fold  of  tissue 
called  the  plica  semilunaris.  This  is  a  remnant  of  that 
third  eyelid  which  in  many  lower  vertebrates,  notably  the 
birds,  is  greatly  developed  and  can  be  drawn  over  the  whole 
eyeball,  inside  the  outer  eyelids  (Plate  97). 

These  vestigial  structures  in  man  have  little  or  no  mean- 
ing until  in  them  we  recognize  the  traces  of  an  earlier  con- 
dition through  which  our  ancestors  have  passed. 

In  human  embryology  there  is  every  indication  that  we 
must  regard  man  as  closely  related  to  the  rest  of  the  ani- 
mal kingdom.  A  little  study  of  the  illustrations  of  the 
embryos  of  man  and  a  number  of  other  vertebrates  will 
bring  out  this  resemblance  in  their  embryology,  and  the 
fact  that  the  human  embryo,  in  the  earlier  stages  of  its 
growth,  has  many  features  which  are  a  reminiscence  of  its 
fishlike  early  ancestors  (Plate  98).  In  the  later  develop- 
ment of  the  human  child,  after  birth,  there  are  a  "number 
of  things  that  are  instructive  in  this  connection.  In  a  baby 
the  spinal  column  has  a  single  curve,  as  it  does  in  the  apes 
and  monkeys,  instead  of  the  S-shaped  curve  seen  in  the 
adult  human  being  (Plate  99).  The  feet  are  held  in  a 
position  characteristic  of  the  apes  (Plate  100).  For  a  few 
weeks  after  birth,  the  child  has  a  remarkably  strong  finger- 
grip,  recalling  the  strength  with  which  the  young  apes  grasp 
the  mother's  hair,  as  she  climbs  with  them  among  the  trees. 
The  young  human  baby  is  able  to  sustain  its  own  weight 
by  its  hands,  and,  when  hanging  thus,  shows  often  a  posi- 
tion of  the  legs  which  is  strikingly  apelike  (Plate  100,  B\ 


1 7o  ORGANIC  EVOLUTION 

The  position  of  the  legs  after  birth  is,  however,  probably 
largely  due  to  the  prenatal  folded  position  of  the  legs. 

We  might  develop  to  an  indefinite  extent  these  points 
of  anatomical  and  embryological  resemblance  between  man 
and  other  vertebrates.  The  character  of  the  evidence,  how- 
ever, has  been  sufficiently  illustrated.  I  know  of  no  scien- 
tific reason  for  separating  man  from  the  rest  of  the  animal 
kingdom  as  regards  the  processes  of  evolution.  His  whole 
structure  shows  that  he  has  arisen  by  differentiation  from 
lower  vertebrates.  We  do  not  understand  all  the  stages 
by  which  his  body  has  been  thus  evolved,  nor  do  we  know 
in  detail  by  what  steps  his  mental  faculties  have  arisen  from 
the  lower  condition  of  mind  seen  in  other  vertebrates;  yet 
we  have,  apparently,  no  reason  for  believing  that  the  method 
of  their  evolution  has  been  different  in  any  fundamental 
regard  from  the  methods  by  which  the  minds  and  bodies 
of  other  animals  have  been  developed.  Comparative  psy- 
chology is  as  yet  in  its  infancy,  and  we  are  not  at  all  pre- 
pared to  discuss  the  relations  between  the  mind  of  man 
and  the  minds  of  lower  animals,  much  less  to  attempt  to 
describe  the  steps  in  the  evolution  of  the  human  mind.  We 
must  wait  a  good  many  years  before  our  curiosity  in  this 
regard  can  be  satisfied.  There  appears,  however,  no  suffi- 
cient reason  for  believing  that  the  development  of  man's 
mind  has  been  anything  other  than  natural  and  in  accord- 
ance with  the  principles  that  apply  in  the  development  of 
the  minds  of  other  species.  So  far  as  we  can  judge,  man 
is  the  result  of  the  same  processes  and  factors  that  have 
produced  the  bees  with  their  wonderful  instincts  and  the 
tiger  with  his  superb  physique. 

Not  only  has   man   been  produced   under  the  influence 


PLATE  97.  -Eyes  of  various  vertebrates,  showing  the  nictitating  membrane  (third  eyelid), 
indicated  by  the  letter  N.  —  From  Romanes'  Darwin  and  After  Darwin,  by  the  courtesy  oi 
The  Open  Court  Publishing  Company. 


PLATE  99.  —  A  and  B.  Diagrams  illustrating  the  curvature  of  the  spinal  column  in  a  human 
infant  (A)  and  an  adult  man  (ff).  The  curvature  of  the  spinal  column  in  an  ape  (C)  resembles 
that  in  the  human  infant.  (Compare  the  upper  figure  in  cut  C  of  this  plate.)  C.  A  group  of 
gorillas,  male,  female,  and  young;  observe  the  position  of  the  feet  in  the  female  and  in  the  young 
gorilla.  —  From  Brehm's  Thierleben. 


PLATE  100.  —  A.  Foot  position  of  a  human  infant.  —  From  Romanes'  Darwin  and  After  Dar- 
win, by  the  courtesy  of  The  Open  Court  Publishing  Company.  D.  Two  human  infants,  ten  and 
thirteen  days  old  respectively,  supporting  their  weight  by  their  hands.  —  From  a  photograph  by 
Dr.  Louis  Robinson,  by  the  courtesy  of  The  Open  Court  Publishing  Company. 


EVOLUTION   OF  MAN  171 

of  the  factors  of  evolution,  he  is  still  subject  to  them  and 
is  still  being  modified  by  them  to-day.  Disease  and  unfavor- 
able climate  kill  those  who  are  unable  to  resist  them,  while 
the  stronger  survive.  Men  fail  in  the  struggle  for  existence 
and  become  submerged  and  disappear.  Natural  selection 
is  constantly  removing  those  who  are  unable  to  resist  the 
pressure  of  the  adverse  conditions  of  life.  This  is  the  same 
process  we  have  seen  among  the  lower  animals  and  the 
plants,  and  has  the  effect  of  making  man  more  fit  for  his 
surroundings  by  eliminating  the  less  adapted. 

Sexual  selection  also  is  operative,  more  so  among  man- 
kind than  in  any  other  group  of  animals.  There  is  closer 
scrutiny  and  more  careful  choice  is  exercised  in  human 
marriage  than  in  the  mating  of  any  of  the  lower  animals. 
There  is  an  important  difference  to  notice.  Among  human- 
kind, at  least  among  more  highly  civilized  men,  choice  in 
marriage  is  based  more  largely  upon  intellectual  and  moral 
attractions  and  less  upon  physical  attractions  than  is  the  case 
among  lower  animals.  Among  lower  forms  sexual  selection 
secures  chiefly  ornamentation  or  fine  voice.  Among  men  it 
is  more  those  of  good  intellect,  of  pleasing  disposition,  of 
right  character,  who  are  chosen;  sexual  selection  thus  serving 
to  increase  and  perpetuate  these  characteristics. 

Segregation  also  is  an  important  factor  in  human  evolu- 
tion. The  fact  that  the  Chinese  live  in  Asia  and  the  negroes 
in  Africa,  has  prevented  intercrossing  between  these  two 
races,  which,  if  it  had  taken  place,  would  have  changed  the 
character  of  both  races.  In  any  community  there  are  many 
important  segregating  factors.  There  is  in  America  a  well- 
nigh  universal  distaste  toward  marriage  between  negroes  and 
Caucasians,  and  this  has  had  an  important  effect  upon  the 


I72  ORGANIC  EVOLUTION 

development  of  the  two  races.  Intermarriage  between  those 
of  different  social  strata  is  unusual,  culture  and  wealth  thus 
effecting  segregation.  Religious  belief  has  had  an  important 
effect  in  causing  segregation  in  marriage.  It  would  be  im- 
possible to  enumerate  all  the  efficient  causes  of  segregation 
among  humankind. 

Let  us  look  a  little  further  at  man's  relation  to  natural 
selection  and  sexual  selection.  First  as  to  natural  selec- 
tion: While  man,  like  all  other  animals,  is  subject  to  natural 
selection,  he  is  less  so  than  any  other  species,  so  far  as  physi- 
cal factors  are  concerned.  Our  great  intellectual  develop- 
ment enables  us  to  escape  from  many  phases  of  the  struggle 
for  existence.  We  build  houses  which  protect  us  from  the 
inclemency  of  the  weather.  We  have  fires  to  protect  us 
from  the  cold  of  winter.  We  cook  our  food,  thus  largely 
escaping  the  internal  parasites  which  so  commonly  infest  the 
lower  animals.  We  have  physicians  who  enable  us  to  sur- 
vive diseases  which  otherwise  would  destroy 'us.  By  cultiva- 
tion of  the  soil  and  by  raising  flocks  and  herds  we  increase 
the  productiveness  of  the  earth,  making  it  support  a  far 
greater  population  than  would  otherwise  be  possible.  \Vhen 
crops  fail  in  certain  localities,  whole  nations  are  saved  from 
extermination  by  the  great  development  of  our  means  of 
transportation,  which  bring  food  from  distant  regions  to  save 
the  starving.  In  thousands  of  ways  we  are  relieved  by  our 
greater  intelligence  from  much  of  the  stress  of  the  struggle 
for  existence.  Natural  selection  plays  a  less  prominent  part 
among  men  than  among  plants  and  the  lower  animals. 

Of  course  this  partial  elimination  of  natural  selection  is  a 
very  great  advantage,  producing  inestimable  good  to  man, 


EVOLUTION  OF  MAN  173 

yet  there  are  disadvantages  as  well.  By  means  of  our  well- 
warmed  houses  we  protect  ourselves  from  rain  and  cold,  and 
thus  save  from  death  many  delicate  ones  who  would  other- 
wise perish.  But  by  preserving  these  weaker  ones  we  allow 
them  to  hand  down  to  the  next  generation  their  weak  consti- 
tution, and  so  the  race  will  average  less  robust  than  it  would 
be  if  the  weak  ones  had  been  allowed  to  succumb  to  the  cold 
and  so  had  never  had  offspring.  Similarly  the  physician 
saves  from  death  many  a  weakling  whose  children  bring 
down  the  average  of  physical  efficiency  in  the  next  genera- 
tion. Physical  deterioration  has  resulted  from  the  partial 
elimination  of  natural  selection.  Invalids  are  rare  among 
the  lower  animals :  they  are  rare  among  savage  races.  How 
common  they  are  among  us!  The  invention  of  spectacles 
has  allowed  our  eyes  to  deteriorate  without  putting  us  at  a 
serious  disadvantage.  The  skill  of  the  dentist  has  tended 
toward  unsound  teeth  for  civilized  man.  Such  instances 
might  be  multiplied. 

One  point  here  should  be  clearly  seen.  Natural  selection 
seeks  the  highest  efficiency  of  the  species  as  a  whole,  and  to 
this  end  sacrifices  innumerable  defective  individuals,  lest  they 
and  their  children  bring  down  the  average  of  efficiency. 
We,  on  the  other  hand,  seek  the  welfare  of  the  individual 
and  preserve  and  cherish  the  weak,  though  we  know  that  by 
so  doing  we  in  the  end  decrease  the  vigor  of  the  race.  Be- 
cause of  our  charitable  and  altruistic  tendencies  we  preserve 
also  the  intellectually  and  morally  weak,  and  thus  cause  a 
certain  intellectual  and  moral  deterioration  in  the  race  aver- 
age. I  believe  this  is  very  largely  compensated  for  by  other 
considerations,  yet  the  deterioration  is  no  less  real. 

A  good  illustration  of  the  effect  of  natural  selection  in 


I74  ORGANIC  EVOLUTION 

connection  with  disease  is  seen  in  the  relation  of  savage 
peoples  to  certain  mild  diseases  prevalent  among  civilized 
races.  Measles  is  not  very  serious  in  civilized  communities. 
It  has  long  been  a  common  disease.  Those,  in  the  past, 
who  were  unable  to  resist  this  disease  have  died ;  and,  as 
it  is  mostly  a  disease  of  children,  they  have  died  before 
reaching  adult  life  and  becoming  parents.  They  have, 
therefore,  not  transmitted  to  the  next  generation  their  consti- 
tution with  its  slight  powers  of  resistance  to  this  disease. 

Many  of  those  children,  on  the  other  hand,  who  have 
been  strong  enough  to  survive  attacks  of  measles  have 
reached  maturity  and  have  handed  down  to  their  children 
something  of  their  natural  ability  to  resist  its  attacks.  There 
has  thus  been  developed  among  civilized  peoples  a  consider- 
able degree  of  power  to  throw  off  this  disease. 

But  among  savage  races,  the  North  American  Indians, 
for  example,  measles  has  often  been  a  fearful  scourge.  It 
has  not  been  prevalent  among  them  for  many  generations, 
as  among  the  white  peoples,  and  they  have  not  acquired 
through  natural  selection  the  ability  to  resist  it.  Therefore, 
it  was  but  natural  that  when  introduced  among  them  it 
should  wipe  out  whole  communities,  slaying  adults  as  well 
as  children. 

Were  vaccination  now  to  be  universally  given  up,  it  is 
possible  that  small  pox  would  be  more  dangerous  than  it 
used  to  be  before  Jenner  found  a  way  to  save  us  from  its 
ravages;  though  perhaps  vaccination  has  not  been  used 
long  enough  to  allow  much  deterioration  in  the  power  of 
resistance  to  small  pox  which  was  to  a  degree  acquired  dur- 
ing those  centuries  when  the  disease  had  free  course. 

How  far  will    the  deterioration  which  results  from    par- 


EVOLUTION  OF  MAN  175 

tially  freeing  ourselves  from  the  action  of  natural  selection 
go?  It  cannot  go  on  indefinitely.  Natural  selection  still 
eliminates  those  who  are  physically  very  defective;  so  also 
sexual  selection  will  operate  against  the  perpetuation  of 
physical  deformity  and  great  physical  weakness.  We  need 
not  fear  the  extermination  of  the  race  through  freeing  our- 
selves from  the  action  of  natural  selection.  I  think,  how- 
ever, that  we  must  anticipate  a  still  further  physical 
deterioration  of  humankind,  not  only  in  such  minor  points 
as  our  teeth  and  eyes,  but  in  all  regards,  invalidism  becom- 
ing more  and  more  prevalent  as  medical  skill  advances. 

There  is  another  profitable  inquiry  as  to  our  relation  to 
natural  selection.  What  is  the  nature  of  our  environment 
to  which  we  must  conform  in  order  to  survive  and  prosper 
and  succeed  in  giving  our  children  favorable  opportunities  ? 
The  environment  of  lower  animals  and  plants  is  made  up 
of  many  elements  that  have  a  bearing  upon  their  lives  — 
climate,  food  and  drink,  enemies,  disease,  etc.  We  have 
the  same  elements  in  the  physical  environment  to  which 
we  have  to  relate  ourselves,  but  in  addition  we  have  another 
factor,  perhaps  as  important  as  any,  namely,  public  opinion. 
Unless  we  conform  to  a  certain  standard  of  intelligence, 
moral  character,  and  good  taste  we  find  ourselves  at  a  dis- 
advantage in  life,  and  have  to  struggle  hard  to  maintain 
ourselves  and  care  for  our  children.  The  man  who  in  any 
or  in  all  of  these  ways  is  far  in  advance  of  his  fellows,  or 
the  one  who  falls  much  below  popular  standards,  feels  the 
pressure  of  life  more  than  he  who  conforms  to  the  popular 
ideas  of  right  character  and  good  taste.  Conformity  to 
public  opinion  is  of  great  importance  if  one  desires  the 


I76  ORGANIC  EVOLUTION 

best   chance    of   survival    for   himself    and    family.       Public 
opinion  is  a  vitally  important  part  of  our  environment. 

It  is  not  only  important  as  regards  natural  selection ; 
it  is  perhaps  even  more  important  in  relation  to  sexual 
selection.  A  man  or  woman,  to  be  desired  as  a  husband 
or  wife,  must,  in  general,  be  one  whose  ideas  of  right  living 
conform  to  those  of  the  community,  one  whose  character 
and  disposition  are  such  as  to  command  respect.  These 
characteristics  have  more  influence  upon  choice  in  marriage 
than  do  merely  physical  characteristics. 

It  may  be  worth  our  while  to  ask  one  further  question. 
Under  present  conditions,  how  is  the  race  to  make  desirable 
progress  ?  How  can  we  influence  the  evolution  of  the  race, 
so  that  it  shall  take  the  right  direction  ?  Notice,  first,  that 
the  very  asking  of  this  question  indicates  an  interesting  con- 
dition. We  can,  to  a  considerable  extent,  control  our  own 
evolution.  The  lower  animals  cannot  do  so.  They  lack 
the  intelligence  which  gives  us  this  power. 

How  shall  we  secure  the  evolution  of  the  race  in  desir- 
able directions  ?  Before  attempting  to  discuss  this  question 
it  is  important  to  distinguish  clearly  between  human  evolu- 
tion and  social  progress.  By  evolution,  as  we  here  use  the 
term,  we  mean  a  change  in  innate  character.  Social  progress 
may  be  secured  by  training  the  individuals  of  each  succeed- 
ing generation  to  higher  and  higher  standards  of  living, 
even  while  no  change  in  the  innate  character  of  the  race 
has  been  brought  about. 

The  distinction  we  would  emphasize  can  be  easily  illus- 
trated. If  a  savage  should  receive  some  suggestion  that 
should  cause  him  to  improve  his  standard  of  living  his 


EVOLUTION  OF  MAN  177 

whole  family  would  be  benefited.  The  son  born  into 'this 
family  would  receive  by  education  the  knowledge  of  the 
better  way  of  living.  He  would,  naturally,  during  his  own 
lifetime,  learn  still  more,  making  the  life  of  his  family  a  little 
more  comfortable  than  was  that  in  his  father's  home.  His 
son  would  therefore  be  born  into  a  more  favorable  family 
environment  than  that  in  which  he  passed  his  own  early 
life.  Thus  from  generation  to  generation,  through  experi- 
ence, the  results  of  which  would  be  handed  on  by  education, 
the  standard  of  living  would  be  improved  in  the  families  of 
the  descendants  of  this  savage.  Great  progress  might  be 
thus  made  without  any  change  in  the  inborn  nature  of  the 
children  from  generation  to  generation. 

Continuing  the  illustration,  we  may  suppose  a  child  of 
the  tenth  (or  thousandth)  generation  to  be  stolen  from  its 
parents  at  birth  and  removed  from  the  improved  family  en- 
vironment, to  be  taken  to  a  primitive  savage  home  similar 
to  that  of  his  savage  ancestor  with  whom  our  illustration 
started.  We  have  no  reason  to  believe  that  under  these 
circumstances  the  higher  culture  of  his  ancestors  for  nine 
generations  would  cause  him  to  lead  any  better  life  than 
if  his  ancestors  had  remained  primitive  savages.  The  nine 
generations  of  advancing  culture  secured  by  education  need 
not  have  produced  any  change  in  innate  character  in  the 
descendants.  The  social  progress  may  have  been  secured 
without  any  real  evolution. 

Social  progress  and  evolution  may,  therefore,  be  very 
different  things.  The  former  is  secured  chiefly  through  the 
transmission  by  education  of  the  knowledge  and  moral  tone 
reached  through  experience,  and  by  the  summation  genera- 
tion after  generation  of  these  increments  of  progress.  Evo- 


I78  ORGANIC  EVOLUTION 

lution  of  the  race,  on  the  other  hand,  is  a  fundamentally 
different  thing.  It  will  be  secured  by  the  same  methods 
which  are  operative  to  produce  evolution  among  the  lower 
animals,  i.e.  through  natural  selection  and  sexual  selection, 
influenced  of  course  by  segregation.  We  have  seen  that  it 
is,  to  say  the  least,  very  doubtful  if  parental  modifications 
are  inherited.  We  have  no  reason  to  believe  that  the 
progress  in  culture,  secured  by  education  in  one  generation, 
will  directly  improve  the  innate  character  of  the  children 
of  the  next  generation. 

Were  the  effects  of  education  inherited,  human  evolution 
should  be  rapid,  but  it  has  been  slow ;  how  slow  perhaps  few 
of  us  realize.  We  speak  with  pride  of  the  advance  in  human 
civilization,  of  our  progress  in  the  arts  and  in  useful  knowl- 
edge, of  the  improvement  in  morals  and  the  growth  of  altru- 
ism, and  this  all  makes  us  blind  to  the  fact  that  since  the 
dawn  of  history  there  has  been  no  very  great  real  evolution 
of  mankind.  We  reach  larger  results  in  the  problem  of  life 
than  did  our  progenitors  five  thousand  years  ago,  but  we  are 
able  to  do  so  because  we  build  upon  their  experience  and 
that  of  all  the  generations  between. 

Have  we  much  greater  innate  powers  ?  Are  we  at  birth 
endowed  with  characters  having  much  higher  possibilities 
and  much  higher  tendencies  physically,  intellectually,  and 
morally  ?  Have  we  to-day  men  of  much  greater  physical 
prowess  than  the  ancient  conquerors  of  the  world,  than  the 
builders  who  constructed  the  monuments  of  Egypt  ?  Have 
we  more  adventurous  spirits  or  more  successful  explorers  than 
the  Phoenicians,  who  without  compass  sailed  the  ancient  seas, 
reaching  the  whole  Atlantic  coast  of  Europe  and  the  British 
Jsles,  also  passing  southward  even  around  the  tip  of  Africa  ? 


EVOLUTION  OF  MAN  179 

Are  there  among  us  to-day  men  of  keener  inventive  genius 
than  the  one  who  first  used  fire,  or  the  inventor  of  the  lever 
or  of  the  wheel,  or  than  the  man  who  first  made  bronze  or 
smelted  ore?  Our  modern  engines  have  been  invented 
screw  by  screw  by  successive  builders,  each  building  upon 
the  others'  work.  Have  we  to-day  men  of  much  larger  legal 
and  social  understanding  than  the  ancient  lawgivers  who 
forged  the  legal  systems  which  still  are  the  basis  of  our  most 
enlightened  governments?  Have  we  poets  whose  genius 
greatly  transcends  that  of  Homer  or  of  the  authors  of  the 
books  of  Job  arid  Ruth  ?  In  aesthetic  appreciation  and  in 
the  power  of  artistic  expression  in  sculpture  and  architecture 
we  are  degenerate  compared  with  the  Greeks. 

Even  in  innate  moral  character  have  we  greatly  advanced  ? 
We  are  learning  the  lesson  of  altruism,  but  are  we  born  with 
a  sturdier  moral  sense  ?  If  we  could  take  a  hundred  thousand 
infants  from  London  or  Chicago  and,  turning  back  the  wheel 
of  time,  place  them  in  the  homes  of  ancient  Babylon,  would 
they  reach  a  higher  standard  of  righteousness  or  of  altruism 
than  their  neighbors  ?  How  little  evidence  we  have  of  real 
evolution  of  mankind  since  the  first  emergence  of  the  race 
from  the  darkness  of  prehistoric  times ! 

Whether  or  not  we  believe  that  man  has  advanced  in 
innate  character  during  the  last  five  or  ten  thousand  years, 
we  can  certainly  say  that  the  advance  has  not  been  rapid. 
The  zoologist  thinks  of  the  problems  of  evolution  in  periods 
of  geologic  time,  not  in  years.  He  sees  decided  change  in 
the  ancestors  of  the  horse,  when  he  compares  the  Eocene 
and  Miocene  fossil  faunas.  He  would  hardly  expect  to  find 
great  progress  in  evolution  indicated  in  the  fossils  found  in 
the  last  few  feet,  say,  of  the  Miocene  strata,  which  would 


!8o  ORGANIC  EVOLUTION 

represent  a  period  of  time  equal  to  the  five  to  ten  thousand 
years  of  human  history. 

Is  it  then  hopeless  ?  Is  there  no  probability  of  securing 
real  advance  for  man  in  innate  character  ?  Must  we  content 
ourselves  with  merely  a  veneering  of  civilization  over  the 
fundamental  savage  nature? 

The  questions  asked  in  the  last  few  paragraphs  force 
themselves  upon  the  attention  of  any  candid  student  of 
human  evolution.  The  author  does  not  claim  to  be  able  to 
furnish  a  complete  answer  to  them,  but  he  would  make  a 
few  suggestions. 

Setting  aside  the  inheritance  of  parental  modifications, 
of  which  we  have  no  evidence,  and  whose  reality  seems  so 
improbable,  we  have  the  two  factors  —  natural  selection  and 
sexual  selection,  aided  by  segregation.  From  the  action  of 
natural  selection  we  in  considerable  measure  escape.  (Com- 
pare pages  169  to  172.)  Even  from  the  action  of  public 
opinion,  one  of  the  most  important  elements  in  our  environ- 
ment, we  in  part  escape  by  our  adaptability.  One  whose 
innate  character  is  unsound  may  be  trained  to  so  conform,  at 
least  outwardly,  to  the  standards  of  the  community  that  he 
will  be  held  in  esteem  and  will  succeed  in  rearing  his  family 
in  conditions  of  comfort.  On  the  other  hand,  a  boy  of  natu- 
rally more  desirable  character  may,  by  wrong  training,  be 
brought  into  such  relation  to  the  community  that  he  will  be 
destroyed.  Survival  in  the  struggle  for  existence  among 
humankind  is  influenced  not  by  innate  character  alone,  but 
by  what  this  character  comes  to  be  through  training.  This 
greatly  complicates  the  problem  of  securing,  through  survival 
of  the  best  adapted,  an  advance  in  innate  character,  i.e.  true 
evolution.  The  plasticity,  or  educability,  of  the  human 


EVOLUTION  OF  MAN  !8i 

being  preserves  him  from  destruction  in  the  struggle  for  life. 
Natural  selection  secures  the  preservation  of  the  more  plas- 
tic, and  this,  in  turn,  makes  it  still  more  difficult  to  secure 
advance  in  innate  character. 

Likewise  sexual  selection,  choice  in  marriage,  among 
humankind  is  based  not  alone  on  innate  character,  but  upon 
what  the  character  has  become  through  training.  This  again 
hinders  advance  in  innate  character  through  sexual  selection. 

But  however  powerful  training  may  be  in  determining 
the  character  of  the  adult  man  or  woman,  still  the  innate 
character  does  count,  and  in  the  long  run  both  natural  selec- 
tion and  sexual  selection  should  tend  to  modify  it.  The 
child  with  weak  body  may  by  training  become  a  strong 
man,  yet,  in  general,  it  is  true  that  the  strong  children 
make  the  strong  men.  So  also  a  child  of  inferior  intel- 
lectual endowments  may  by  proper  culture  become  a  man 
of  considerable  intellectual  development,  yet  on  the  whole 
it  is  true  that  men  of  high  mental  power  were  probably 
boys  of  good  intellectual  capacities. 

We  know  less  about  innate  moral  character,  still  it  seems 
to  be  true  that  men  differ  greatly  in  their  innate  moral  sound- 
ness and  moral  sensitiveness.  There  is  much  evidence  in 
favor  of  the  belief  that  one  of  mediocre  moral  endowments 
may  by  proper  training  become  a  man  of  moral  power,  yet 
here  again  it  seems  to  be  true  that,  in  general,  innate  moral 
capacities  are  correlated  with  high  moral  attainments. 

If,  therefore,  there  is  such  a  general  correlation  between 
innate  capacities  and  attainments,  whether  physical,  intellec- 
tual, or  moral,  it  must  follow  that,  in  so  far  as  natural  and 
sexual  selection  operate,  they  will  tend  gradually  to  modify 
innate  character  in  these  three  aspects. 


!82  ORGANIC  EVOLUTION 

One  further  inquiry  must  be  made:  Is  mutation  occurring 
among  humankind  ?  If  not,  no  evolution  is  possible.  We 
can  hope  then  only  to  raise  mankind  nearer  to  the  level  of  our 
present  best,  and  cannot  hope  to  raise  this  best  to  a  still 
higher  level.  Variation  among  men  has  not  as  yet  been 
thoroughly  studied  from  the  new  point  of  view  and  we  do 
not  know  to  what  extent  mutation  is  at  present  occurring. 
It  has  occurred  in  the  past,  as  is  indicated  by  the  divergence 
between  races,  and  we  have  many  conspicuous  instances  of 
recent  mutation,  as,  for  example,  the  origin  of  the  Haps- 
burg  upper  lip  which  has  been  so  persistent  in  the 
house  of  Austria.  It  seems  altogether  probable  that 
mutations  in  both  physical  and  psychical  characters  are 
occurring,  but  just  how  prevalent  they  are  can  be  told  only 
by  much  further  study. 

Believing  then  that,  in  spite  of  all  deterrent  influences, 
both  natural  selection  and  sexual  selection  do  operate  slowly 
to  produce  modification  in  innate  character,  let  us  ask  again 
the  question :  Can  we  so  control  this  evolution  that  it  will  be 
in  desirable  directions,  and,  if  so,  how  can  it  be  controlled? 

Let  us  elevate  the  standards  of  public  opinion  by  every 
means  in  our  power,  and  then  natural  selection  and  sexual 
selection,  which  are  greatly  influenced  by  public  opinion, 
will  secure  the  evolution  of  the  race.  The  progress  will  be 
slow,  painfully  slow,  but  it  will  be  real.  This  does  not  mean 
that  we  shall  cease  trying  to  improve  individuals.  Each 
individual,  who  is  led  to  a  more  desirable  attitude  toward 
life,  will  act  as  leaven  in  the  community  in  which  he  lives, 
raising  somewhat  the  standards  of  the  whole  community.  I 
believe  that  in  the  continued  influence  of  Jesus  we  find  the 
greatest  force  tending  to  the  improvement  of  the  individual 


EVOLUTION  OF  MAN  183 

character  and  to  the  elevation  of  public  opinion,  and  so  to 
the  evolution  of  mankind  in  desirable  directions. 

Improvement  in  social  conditions,  even  though  reached 
through  improved  education,  generation  after  generation, 
rather  than  by  advancing  the  innate  qualities  of  the  race,  is 
of  course  a  most  worthy  object  for  which  to  labor,  and  it  is 
comforting  to  find  that  there  is  hope  that  such  efforts  may, 
in  the  course  of  thousands  of  years,  improve  also  the  innate 
fibre  of  the  race  through  the  effect  which  the  advance  of 
public  opinion  will  have  upon  natural  and  sexual  selection. 
To  those  who  have  faith  in  immortality,  work  for  the 
improvement  of  the  individual  assumes  added  importance 
irrespective  of  its  relation  to  evolution. 

We  have  referred  to  the  relative  importance  of  sexual 
selection,  choice  in  marriage,  in  the  evolution  of  mankind. 
This  point  deserves  practical  emphasis.  In  choosing  a  wife 
a  man  is  selecting  the  mother  of  his  children  as  well  as  a 
companion  for  himself,  and  he  should  think  as  much  and 
more  of  those  qualities  that  tend  to  make  a  good  mother 
as  of  those  which  will  make  an  agreeable  companion.  A 
woman  in  accepting  the  responsibilities  of  marriage  should 
look  forward  to  her  children's  welfare  and  think  as  much  of 
the  father  she  is  giving  to  her  children  as  of  the  husband  she 
is  accepting  for  herself.  I  believe  that  love  is  the  chief  con- 
sideration, and  that  it  would  be  a  serious  misfortune  to  have 
this  relegated  to  the  background,  as  it  is  among  so  many 
peoples.  Fortunately  this  seems  unlikely  ever  to  occur  in 
America.  Yet  all  important  as  is  love,  the  essential  foun- 
dation in  marriage,  it  is  not  the  only  thing.  The  welfare 
of  the  coming  generation  is  bound  up  in  the  choices  in 
marriage  of  the  present  generation,  and  this  fact  should 


!84  ORGANIC  EVOLUTION 

never  be  forgotten.  There  are  those  who  because  of  physi- 
cal, intellectual,  or  moral  disability  should  not  be  parents, 
and  there  is  need  of  a  general  public  sentiment  which  will 
recognize  it  as  a  sin  against  society  for  such  to  seek  their 
own  happiness  in  marriage  when  unable  properly  to  meet 
the  responsibilities  of  marriage,  of  which  the  bearing  and 
rearing  of  children  are  a  vital  part.  In  spite  of  the  senti- 
ment in  much  of  our  poetry,  our  novels,  and  the  drama  that 
love  is  supreme  and  therefore  all  else  should  be  sacrificed  for 
it,  it  is  really  selfish  and  evil  to  regard  only  present  happi- 
ness and  forget  the  coming  generation. 

I  believe  that  gradually  this  ideal  of  responsibility  to  the 
race  will  work  its  way  more  and  more  into  the  social  mind, 
and  a  larger  thoughtfulness  before  entering  into  marriage 
will  result.  It  will  come  first  in  our  great  literature,  but  it 
will  leaven  all  society  in  time.  More  strict  statutory  limita- 
tions upon  marriage  may  ultimately  be  wise,  but  these  will 
not  now  secure  the  desired  result.  This  will  be  reached 
only  through  a  larger  general  recognition  of  the  responsi- 
bilities in  marriage,  and  the  worthiness  and  beauty  of 
unselfishness  here  as  everywhere  else.  Thus,  in  time,  choice 
in  marriage  may  do  much  to  counteract  the  hurtful  influence 
of  having  freed  ourselves  from  the  stress  of  the  struggle  for 
existence. 

One  good  influence  upon  choice  in  marriage  is  being 
felt  through  a  comparatively  recent  change  in  the  lives  of 
women,  outdoor  sports  and  outdoor  life  in  general  having 
become  so  much  more  popular.  Riding,  tennis,  golf,  the 
bicycle,  bird  study,  nature  observation,  and  the  love  of  nature, 
all  are  tending  to  take  more  and  more  women  into  the  open 
air.  These  things  are  perceptibly  changing  the  ideas  of 


EVOLUTION  OF  MAN  !85 

what  constitutes  attractiveness  in  a  woman.  It  is  now 
somewhat  the  case,  and  seems  likely  to  be  more  largely  true, 
that  the  girl  who,  because  of  physical  incapacity,  cannot  share 
in  this  vigorous,  healthful,  outdoor  life,  will  be  at  a  social  dis- 
advantage. This  is  but  one  way  of  saying  that  considerations 
of  physical  vigor  will  increasingly  influence  choice  in  marriage, 
and  this,  of  course,  will  be  for  the  welfare  of  the  race. 

It  is  interesting  to  think  what  might  be  the  result  if  there  were  started  a 
sect  in  which  careful  choice  in  marriage,  under  the  advice  of  those  most  able 
to  discern  hereditary  tendencies,  should  be  regarded  as  a  sacred  obligation, 
looking  toward  an  increasing  perfection  of  the  race  in  all  respects,  physical, 
intellectual,  and  moral.  It  would  probably  be  easy  thus  to  raise  "human 
stature,  to  very  greatly  increase  muscular  power  and  agility,  to  very  largely  do 
away  with  invalidism,  to  increase  the  mental  capacity  to  an  indefinite  extent,  and 
at  first  thought  it  would  seem  easy  to  secure  a  race  with  finer  and  firmer  moral  fibre, 
for  probably  man  is  mutating  in  all  these  regards.  Yet  there  would  be  serious 
difficulties  in  the  way.  This,  which  is  the  logical  goal  of  socialism,  would  be 
likely  to  mar  the  beauty  of  family  life,  which  is  dependent  upon  a  peculiar 
mutual  attraction  between  individuals,  that  cannot  be  dictated.  The  time 
may  possibly  come  when  individuals  will  so  cordially  recognize  their  responsi- 
bility for  the  advancement  of  the  race  that  choice  in  marriage  will  look  to  the 
welfare  of  the  race  as  a  whole,  rather  than  to  that  of  the  family,  as  the  chief 
goal ;  but  this  will  not  come  in  our  day  or  before  there  has  been  wrought  in 
men  a  most  far-reaching  change  in  life  ideals.  We  have  reached  the  stage  in 
which  there  is  more  or  less  general  recognition  of  the  fact  that  in  marriage 
the  welfare  of  the  family  rather  than  that  of  the  individual  should  be  sought 
by  all  intelligent  and  right-minded  persons ;  but  it  seems  impossible  that  the 
welfare  of  the  race  can  ever  be  secured  at  the  sacrifice  of  the  beauty  of  the 
family  life ;  and  it  is  a  question  whether  the  advancement  of  the  race  physi- 
cally, intellectually,  and  morally,  by  choice  in  marriage,  directed  chiefly  to 
that  end,  can  be  secured  without  lessening  the  beauty  of  family  life.  The 
elevation  of  general  standards  of  opinion  as  to  what  constitutes  attractiveness 
in  a  man  or  woman,  so  that  these  shall  include  physical,  intellectual,  and 
moral  soundness  and  beauty,  will  cause  choice  in  marriage  to  operate  for  the 
perfection  of  the  race  along  these  lines,  desire  and  duty  combining  to  pro- 
mote the  progress  of  the  race.  It  is  apparently  hopeless  to  accomplish  much 
in  this  direction  by  cultivating  the  sense  of  duty  at  the  expense  of  love.  A 


!86  ORGANIC  EVOLUTION 

family  founded  upon  the  sense  of  duty  and  not  upon  love  would  not  be  the 
best  soil  in  which  to  cultivate  the  most  beautiful  elements  of  character. 

An  objection  might  be  made  to  the  idea  of  evolution 
among  men  through  the  action  of  sexual  selection,  similar  to 
that  which  was  made  to  the  effectiveness  of  sexual  selection 
among  lower  animals,  —  namely  that,  to  secure  evolution 
in  the  desired  direction,  public  opinion  must  be  so  strong 
that  few  but  those  possessing  the  desirable  qualities  shall 
succeed  in  marrying,  a  condition  of  whose  coming  we  see  no 
present  signs.  But  this  objection  is  really  without  weight. 
If  men  of  fine  stamina,  physically,  intellectually,  and  morally, 
seek  to  marry  and  are  accepted  by  women  of  similar  charac- 
ter, their  children  will  in  the  end  predominate  over  the  off- 
spring of  the  physically,  intellectually,  and  morally  weak,  no 
matter  how  many  of  the  latter  may  marry,  or  how  large  be 
their  families.  Comparatively  few  people  are  living  to-day 
who  will  have  any  descendants  a  thousand  years  from  now, 
and  these  are  men  of  vigor  and  soundness,  not  only  physi- 
cally, but  intellectually  and  especially  morally,  for  nothing 
will  more  surely  bring  a  line  of  descendants  to  its  close 
than  moral  unsoundness.  If  the  best  among  us  should 
marry  the  best,  and  generation  after  generation  keep  the 
strain  free  from  taint  of  weakness,  real  evolution  in  desirable 
directions  would  be  much  more  rapid.  We  need  a  more 
wholesome  ideal  of  character,  so  that  we  shall  delight 
in  real  strength,  delight  in  men  and  women  who  in  each 
phase  of  their  character  have  stamina  and  power.  Strength- 
ening this  ideal  and  spreading  it  among  men  is  the  hope  of 
evolution  into  larger  manhood.1 

1  Kellicotfs  The  Social  Direction  of  Human  Evolution  is  a  valuable  and  readable 
introduction  to  the  scientific  study  of  the  problem  of  securing  a  finer  human  race. 


GENERAL    CONSIDERATIONS  187 

SOME   GENERAL   CONSIDERATIONS 

In  closing  this  discussion  of  evolution  let  us  emphasize 
three  general  considerations.  First,  we  should  remember 
that  natural  selection,  the  great  factor  in  evolution,  produces 
adaptation  to  the  conditions  of  the  environment,  and  that 
this  does  not  by  any  means  always  imply  an  advance  in  com- 
plexity of  organization  in  plants  and  animals,  or  greater 
development  of  the  mind  in  animals.  On  the  contrary, 
degeneration,  in  the  sense  of  simplification,  often  results 
from  the  action  of  natural  selection.  To  make  this  point 
more  vivid,  let  us  look  at  an  example  of  extreme  degenera- 
tion, so  far  as  complexity  of  structure  is  concerned,  and  see 
how,  by  its  changed  character,  the  animal  in  question  is 
more  perfectly  adapted  to  the  environment  it  has  chosen, 
and  is  thus  benefited. 

Among  the  simpler  Crustacea,  in  the  same  group  with 
the  common  ship-barnacles  and  goose-barnacles,  there  is  a 
genus  of  parasitic  forms  called  Sacculina.  These  are  fre- 
quently parasitic  upon  the  common  crab.  When  seen  upon 
the  crab  they  appear  to  be  little  more  than  soft  bags  full 
of  eggs,  and  no  one  would  suppose  that  they  were  in  reality 
Crustacea  and  related  to  the  crab  itself  (Plate  101,  C.  Sacc.}. 
They  show  no  hard  outer  covering,  such  as  is  seen  in  all 
normally  developed  Crustacea,  and  from  which  the  group 
derives  its  name.  They  have  no  jointed  legs  as  do  other 
Crustacea.  There  is  nothing  in  their  adult  anatomy  to 
suggest  that  they  are  Crustacea.  No  one  would  think  for 
a  moment  of  so  classifying  them,  were  it  not  for  their 
embryology,  which  clearly  shows  that  they  are  descended 
from  forms  which  closely  resemble  goose-barnacles.  In  the 


1 88 


ORGANIC  EVOLUTION 


course  of  their  embryology  we  see  a  larva,  which  is  like  that 
usually  found  in  the  Crustacea,  the  so-called  Nauplius 
(Plate  10 1,  A).  This  is  followed  by  another  stage  in 
which  we  see  the  animal  resembles  Cypris,  one  of  the 
Ostracoda,  a  group  of  lowly  developed  Crustacea  (Plate 
1 01,  B}.  Soon  the  little  Sacculina  larva  passes  through 
this  stage  and  comes  to  a  higher  condition  when  it  is  practi- 


A  B  c 

FIG.  46.  —  Development  of  Sacculina  carcini. 

A.  Larva  which  has  just  become  attached  to  the  base  of  a  hair  (6)  on  the  surface  of  a  crab- 
It  is  throwing  off  its  legs  and  part  of  its  body.  B,  C,  D.  Further  stages  in  the  degeneration  of 
the  Sacculina  larva  while  attached  to  the  outer  surface  of  the  crab.  [After  DELAGE.] 

cally  a  little  goose-barnacle.  Now  it  leaves  its  independent, 
free-swimming  life  and  becomes  attached  to  a  crab,  or 
occasionally  some  other  animal  (Fig.  46,  A).  Living  at- 
tached to  the  crab,  as  it  does,  the  parasite  has  no  use  for 
legs  or  any  locomotor  organs,  and  these  are  cast  off.  Sense 
organs  are  not  needed,  and  these  are  lost.  There  being  no 
sense  organs  and  no  muscles  to  be  controlled,  the  useless 
nervous  system  becomes  very  much  simplified  (Fig.  46). 

Apparently  because  of  the  protection  thus  afforded,  the 
Sacculina    penetrates   now  within    the    tissues    of    the  crab, 


Sacc 


Pl-ATE  101.  —  Sacculina. 

A.  Its  nauplius  larva.  B.  The  Cypris  stage  in  its  development.  C.  The  adult  Sacculina  para- 
sitic upon  a  crab,  to  the  under  side  of  whose  abdomen  it  is  attached,  and  whose  body  is  pene- 
trated in  all  directions  by  the  root-like  processes  of  the  Sacculina.  [From  WEISMANN,  after 
DELAGE.]  D.  A  larva  which  has  crawled  into  the  interior  of  the  body  of  a  crab  where  it  is 
rapidly  growing  as  it  feeds  from  the  blood  of  the  crab;  it  is  now  an  almost  shapeless  mass  of 
cells.  £.  A  section  through  a  mature  Sacculina.  Most  of  its  body  has  been  pushed  out  from 
the  inside  of  the  crab  and  now  protrudes  to  the  exterior.  There  are  no  appendages  or  sense 
organs,  and  the  nervous  system  (g)  is  greatly  reduced.  The  body  contains  little  but  the  ovaries 
(ov.)  and  testes  (t.)  full  of  eggs  and  spermatozoa.  [After  DELAGE.] 


GENERAL    CONSIDERATIONS  189 

becoming  an  internal  parasite  instead  of  an  external  parasite 
as  at  first  (Plate  101,  D}.  While  thus  parasitic  it  gets  its 
food  from  the  blood  of  the  crab,  which  of  course  contains 
much  digested  food  ready  to  be  assimilated.  As  digested  food 
is  supplied  for  its  use,  the  Sacculina  has  no  need  of  digestive 
organs  of  its  own,  and  consequently  these  disappear.  Here, 
within  the  tissues  of  its  host,  relieved  of  all  need  of  gathering 
or  digesting  its  own  food,  and  freed  from  the  necessity  of 
moving  about  from  place  to  place  by  its  own  energy,  it  has  an 
abundant  amount  of  energy  to  devote  to  its  growth  and  to  the 
formation  and  maturing  of  its  reproductive  elements. 

The  Sacculiua  soon  becomes  little  more  than  a  bag  of 
eggs  and  spermatozoa  held  together  by  a  little  soft  tissue 
which  surrounds  these  germ  cells.  In  this  condition,  appar- 
ently to  allow  of  its  growth  to  still  larger  size,  it  begins  to 
protrude  from  the  body  of  the  crab,  becoming  in  the  end  a 
bag  of  considerable  size  held  to  the  crab  by  root-like  pro- 
cesses that  penetrate  through  the  shell  and  into  the  body  of 
the  crab,  and  take  up  nourishment  from  its  blood  (Plate 
10 1,  E).  Soon  the  Sacculina  bursts  and  the  eggs  are  set  free, 
and  each  starts  upon  a  new  cycle  of  development  similar  to 
that  described. 

Life  under  the  conditions  of  parasitism  is  very  easy,  and 
it  is  no  wonder  that  many  animals  and  plants  have  been 
adapted  to  such  life.  Since  many  organs  essential  to  the 
welfare  of  self-dependent  animals  are  useless  to  parasitic 
forms,  we  find  that  parasitism  is  usually  associated  with  the 
loss  of  these  useless  organs ;  or,  in  other  words,  we  can  say 
that  parasitism  results  in  simplification.  We  have  quoted 
an  extreme  instance  of  simplification.  There  are  other 
cases  of  as  great  simplification  of  structure,  but  in  most 


190 


ORGANIC  EVOLUTION 


instances  the  degeneration  is  not  so  pronounced.  Phe- 
nomena of  degeneration,  however,  are  not  observed  only  in 
parasitic  forms  but  are  very  general,  and  animals  which  as  a 
whole  are  not  degenerate,  usually  have  some  of  their  organs 
degenerate.  In  our  own  bodies  are  many  such  degenerate 
organs  (skin  muscles,  except  over  the  face ;  ear  muscles, 
Plate  96;  tail,  coccyx,  Plate  91  ;  third  eyelid,  Plate  97;  hair 
of  body,  Plate  93 ;  vermiform  appendix,  Plate  96,  B ;  and  a 
hundred  others).  Many  phenomena  of  simplification  are  just 
as  much  the  result  of  natural  selection  as  are  the  phenomena 
of  increasing  complexity  of  structure.  Natural  selection 
brings  about  more  perfect  adaptation  to  the  conditions  of 
life,  no  matter  whether  this  more  perfect  adaptation  be 
secured  through  simplification  or  through  elaboration. 

Change  in  its  conditions  of  life  may  render  certain  struc- 
tures in  an  organism  useless,  so  that  natural  selection  will 
cease  to  keep  the  structures  up  to  their  former  highly  devel- 
oped condition.  Simplification  may  therefore  be  due  either 
to  cessation  of  the  action  of  natural  selection  when  an  organ 
has  become  useless  or  to  the  direct  action  of  natural  selec- 
tion in  cases  in  which  simplification  is  advantageous. 

A  second  principle  of  great  importance,  and  one  we  have 
already  emphasized,  is  that  natural  selection  secures  the  wel- 
fare of  the  species  and  not  that  of  the  individual,  unless  the 
welfare  of  the  individual  happens  to  be  promoted  by  that 
which  brings  about  the  welfare  of  the  species.  Nature  is 
socialistic,  not  individualistic,  in  the  processes  of  evolution, 
and  this  statement  applies  to  her  relations  to  humankind 
as  well  as  to  her  relations  to  plants  and  the  lower  animals. 
Those  races  whose  ideals  of  life  are  such  as  to  brino-  men 


GENERAL    CONSIDERATIONS  191 

into  the  most  advantageous  relations  to  their  environment 
will  in  the  end  prevail.  But  by  the  most  advantageous 
relations  to  the  environment,  we  mean  such  relations  as  will 
most  effectively  secure  the  perpetuation  and  increase  in  num- 
bers of  the  race,  and  do  not  mean  to  imply  any  moral  signifi- 
cance. It  is  interesting,  however,  to  observe  that  nothing 
promotes  the  preservation  and  increase  of  mankind  more 
than  good  morals,  the  foundation  for  which  is,  in  great  part, 
respect  for  the  general  welfare. 

A  third  general  consideration:  There  are  two  great 
factors  in  the  processes  of  organic  evolution, —  first,  the 
nature  of  the  organism ;  and,  second,  the  character  of  the 
environment  and  its  relation  to  the  organism.  Of  the  lat- 
ter, the  character  of  the  environment  and  its  relation  to 
the  organism  through  the  struggle  for  existence  and  in 
other  ways,  we  know  much.  Of  the  intimate  nature  of  the 
organism,  however,  we  as  yet  know  but  little.  We  do  not 
even  know  whether  the  life  processes  are  conducted  in 
accordance  with  the  ordinary  principles  of  chemistry  and 
physics,  or  in  conformity  to  some  more  subtle  "  vital "  prin- 
ciples. There  are  many  questions  which  we  are  unable  to 
answer  because  we  do  not  understand  the  intimate  nature 
of  living  things.  Are  there  inherent  tendencies  in  the  or- 
ganism, leading  it  to  evolve  in  certain  directions  rather 
than  in  others,  as  St.  George  Mivart  contended,  or  is  its 
evolution  controlled  by  the  needs  created  by  the  character 
of  the  environment?  Such  questions  are  as  yet  beyond 
our  ken,  and  we  have  no  present  prospect  of  soon  being 
able  to  answer  them.  It  is  possible  that  our  knowledge  of 
evolution  may  very  materially  advance  when  our  knowledge 
of  the  life  processes  of  living  things  becomes  more  intimate. 


APPENDIX    I 


TRENDS    IN   EVOLUTION 

THE  possibility  of  the  existence  of  definite  trends  in  certain  species  leading 
them  to  evolve  in  certain  directions  rather  than  in  others  is  indicated  by  at 
least  two  sets  of  phenomena. 

I  have  referred  (page  40)  to  the  fact  that  we  have  some  quite  complete 
series  of  fossils  in  which  are  seen  gradual  modification  of  structure,  the  several 
steps  of  the  modification  being  so  slight  as  to  be  of  doubtful  "selection  value. " 
Plate  46  shows  such  a  series  in  the  fossil  horses,  and  Fig.  26,  page  108,  shows 
an  even  more  instructive  series  of  fossil  Paludina  shells.  It  is  difficult  to  be- 
lieve that  the  gradual  transformation  of  the  latter  was  due  to  some  advantage 
from  the  possession  of  a  rugose  shell,  an  advantage  sufficient  to  cause  the 
"selection"  of  each  slightly  more  corrugated  variety.  This  series  of  shells 
seems  to  suggest  an  inherent  trend  toward  greater  rugosity. 

Recent  studies  of  variation  have  shown  that  inherent  tendencies  toward 
modification  in  particular  directions  do  exist  in  at  least  one  species.  De  Vries 
in  Amsterdam,  and  MacDougal,  at  the  New  York  Botanical  Gardens,  in 
their  careful  and  extensive  experiments  in  rearing  an  evening  primrose  ((Eno- 
thera  lamarckiana) ,  found  that  the  mutants  which  arose  were  of  certain  definite 
types  and  that  these  same  types  appeared  generation  after  generation  in  con- 
siderable numbers  (compare  page  18).  De  Vries  found  seven  mutants; 
MacDougal,  fourteen. 

In  the  Amsterdam  garden  the  mutant  albida  appeared  in  four  generations 
from  lamarckiana  parents,  previous  to  1902,  15  albida  appearing  in  one  gen- 
eration, 25  in  another,  n  in  another,  and  5  in  another.  The  mutant  nanella 
appeared  5  times  in  one  generation,  and  in  other  generations,  respectively, 

193 


194 


APPENDIX 


3,  60,  49,  9,  ii,  and  21  times.     The  mutants  lala,  oblonga,  rubrinervis,  and 
scintillans  appeared  frequenty. 

In  the  fourth  generation  along  with  14,000  lamarckiana  plants  there  ap- 
peared 41  gigas,  15  albida,  176  oblonga,  8  rubrinervis,  60  nanella,  63  lata,  and 
i  scintillans,  all  bred  from  lamarckiana  seed.  In  ,he  fifth  generation,  simi- 
larly bred  from  pure  lamarckiana  seed,  among  8000  lamarckiana  plants  were 
found  25  albida,  135  oblonga,  20  rubrinervis,  49  nanella,  142  lata,  and  6  5«w- 
tillans.  In  the  fourth  generation  one  plant  in  80  was  oblonga.  In  the  fifth 
generation  one  plant  in  60  was  oblonga.  De  Vries  himself  says,  "A  [par- 
ticular mutation]  therefore,  is  not  born  only  a  single  time,  but  repeatedly, 
in  a  large  number  of  individuals  and  during  a  series  of  consecutive  years." 

The  mutant  oblonga  differs  from  the  parent  species,  lamarckiana,  not  in 
a  single  feature,  but  in  an  elaborate  complex  of  characters.  The  other  mutants 
likewise  are  distinguished  from  lamarckiana  by  a  complex  of  characters  rather 
rhan  by  a  single  feature. 

The  mutations  can  hardly  be  entirely  fortuitous  if,  for  several  generations, 
out  of  every  thousand  offspring  of  pure  lamarckiana  parents,  there  appear 
more  than  ten  plants  marked  by  the  particular  complex  group  of  characters 
which  designate  oblonga.  Were  oblonga  demarcated  from  lamarckiana  by 
but  a  single  character  it  would  be  remarkable  to  find  it  appearing  repeatedly 
and  in  such  numbers.  When  we  remember  that  it  is  defined  by  an  extensive 
series  of  characters  differentiating  it  from  lamarckiana  and  from  all  other 
mutants  observed,  are  we  not  led  to  the  conclusion  that  mutation  in  (Eno- 
thera  lamarckiana  is  not  wholly  fortuitous,  but  is  to  a  degree  predetermined, 
that  there  is  some  tendency  to  the  production  of  the  oblonga  and  other  types 
in  numbers  much  greater  than  would  be  secured  by  purely  fortuitous  and 
indeterminate  mutation  ? 

It  seems  of  much  interest  that  the  evidence  from  paleontology,  so  long 
emphasized  by  Osborn  and  other  American  students,  in  favor  of  determinate 
variation  (or  mutation)  should  be  borne  out  by  such  careful  observations  as 
those  of  De  Vries  in  so  different  a  field  of  research. 

It  is  possible  that  (Enothera  lamarckiana  is  a  hybrid  and  that  its  mutation 
is  due  to  its  hybrid  character.  I  know  of  nothing,  however,  to  indicate  that 
this  is  the  case. 

These  observed  phenomena  of  determinate  mutation  suggest  an  explana- 


APPENDIX 


195 


tion  of  such  a  series  of  fossils  as  we  see  in  the  horse  or  the  Slavonian  Paludince. 
If  mutations  tend  to  occur  more  in  certain  particular  directions  than  in  others, 
then  unless  these  mutants  are  of  a  disadvantageous  character,  so  as  to  be 
destroyed  by  natural  selection,  there  will  likely  ensue  a  modification  of  the 
species  in  the  direction  of  these  mutants.  It  makes  no  difference  whether  or 
not  we  understand  the  nature  and  cause  of  such  a  tendency  to  mutation  in 
particular  directions ;  the  fact  that  such  tendencies  do  exist,  if  it  be  a  fact, 
must  affect  evolution. 

I  believe  that  certain  phenomena  of  paleontology  and  a  few  observations 
of  mutation  indicate  the  existence  in  some  species  of  such  trends  to  modifica- 
tion in  particular  directions.  It  is  by  no  means  probable  that  such  trends 
exist  in  all  species.  For  all  we  know,  they  may  arise  in  a  species  and  persist 
for  a  time  and  then  disappear.  We  greatly  need  careful,  extended,  tabulated 
observations  upon  the  mutation  of  many  species  to  see  if  mutation  is  fortuitous, 
occurring  equally  in  all  directions,  or  if,  on  the  other  hand,  the  mutations  tend 
to  group  themselves  and  to  be  more  numerous  in  certain  directions  than 
in  others.  The  observations  upon  CEnothera  lamarckiana  are  very  suggestive, 
but  are  hardly  extensive  enough  to  give  a  secure  foundation  to  a  theory  of 
inherent  trends  in  evolution.  (De  Vries'  Plant  Breeding  [1907]  describes 
determinate  mutation  in  numerous  species.) 

GERMINAL   SELECTION 

Weismann,  in  some  of  his  more  recent  writings,  has  urged  that  such  trends 
do  exist,  and  by  his  theory  of  "germinal  selection"  he  has  endeavored  to 
explain  their  persistence.  Weismann  believes  that  all  the  organs  of  the  adult 
are  represented  in  the  egg  and  spermatozoan  by  minute  protoplasmic  par- 
ticles which,  as  development  proceeds,  grow  up  each  into  its  corresponding 
organ.  The  evidence  in  favor  of  this  conception  is  necessarily  theoretical 
more  than  observational,  and  can  hardly  be  stated  in  the  space  at  our  disposal. 
Those  interested  can  find  Weismann's  own  treatment  of  the  subject  in  his 
book  The  Germ  Plasm  and  his  essay  Germinal  Selection,  also  in  his  new 
work  The  Evolution  Theory. 

Having  postulated  this  high  degree  of  organization  in  the  germ  cells,  each 
part  representing  a  particular  future  organ,  Weismann  proceeds  to  attribute 
to  these  several  determinants,  as  he  calls  them,  an  active  struggle  for  food. 


I96  APPENDIX 

He  says  with  Wilhelm  Roux  that  just  as  animals  contend  with  other  animals 
for  food,  so  the  organs  in  the  body  of  any  one  animal  contend  with  each  other 
for  food,  each  taking  what  it  can  get,  the  stronger  organs  (nutritionally) 
getting  most,  the  weaker  faring  more  poorly.  He  carries  this  principle  even 
further  and  says  that  the  parts  of  a  single  cell  are  engaged  in  a  similar  rivalry 
for  food  and  that  in  the  germ  cells  the  determinants  thus  struggle  with  each 
other  for  nutrition.  Finally  he  suggests  that  when  any  determinant  acquires 
an  advantage  in  this  contest  for  food  its  success  will  give  it  added  vigor,  en- 
abling it  to  become  a  still  more  successful  rival  to  its  neighboring  determinants. 
The  effect  will  be  cumulative  and  generation  after  generation  the  favored 
determinants  will  continue  to  increase  in  vigor.  Now,  as  each  determinant 
gives  rise  to  some  particular  portion  of  the  adult,  that  part  of  the  adult  will 
be  modified  step  by  step  as  its  determinant  becomes  more  favored.  The  effect 
upon  the  favored  determinant  in  the  germ  tends  to  be  cumulative,  its  success 
increasing  the  more  vigorous  it  becomes,  and  similarly  the  modification  of  the 
adult  will  steadily  increase.  In  this  way,  Weismann  believes,  the  suggested 
trends  in  evolution  have  arisen  and  persisted. 

The  theory  is  not  so  fanciful  as  this  bald  statement  would  make  it  seem. 
It  is  certainly  well  worth  consideration  from  any  one  who  has  genuine  interest 
in  evolutionary  problems. 

PLASTICITY,  "ORGANIC   SELECTION" 

If  such  trends  in  evolution  exist,  they  suggest  an  interesting  consideration 
in  connection  with  the  plasticity  of  the  individuals  of  certain  species.  We 
have  already  seen  (pages  27,  28,  and  177)  that  the  ability  of  organisms  to  adapt 
themselves  during  their  lifetime  to  conditions  of  disadvantage  may  enable 
them  partially  to  escape  from  the  stress  of  the  struggle  for  existence  and  per- 
sist when,  if  less  plastic,  they  would  be  destroyed.  I  cannot  quite  agree 
with  Morgan,  Osborn,  and  Baldwin  in  the  emphasis  they  have  laid  upon 
this  accommodation  of  the  individual  as  a  guide  to  the  course  of  evolution 
by  natural  selection.  But  if  it  be  true  that  trends  to  evolution  in  particular 
directions  occasionally  arise  in  certain  species,  it  is  conceivable  that  the  adapta- 
bility of  the  individual  members  of  a  species  might  tide  the  species  over  a  period 
of  disadvantageous  environmental  conditions,  giving  time  for  some  new  and 


APPENDIX 


I97 


advantageous  trend  to  appear.     Such  an  effect  is  not  only  conceivable ;  it  seems 
not  unlikely  that  it  may  have  been  important. 

I  have  said  above  that  I  cannot  quite  agree  with  Morgan,  Osborn,  and 
Baldwin  in  the  emphasis  they  have  laid  upon  the  accommodation  of  the  in- 
dividual as  a  guide  to  the  course  of  evolution  by  natural  selection.  Before 
commenting  further  on  this  suggestion,  let  me  quote  in  part  Professor  Conn's1 
statement  of  the  principle  of  "organic  selection,"  as  this  factor  in  evolution 
has  been  called :  — 

"The  essence  of  the  theory  of  organic  selection  is,  that  these  acquired 
variations  will  keep  the  individuals  in  harmony  with  their  environment,  and 
preserve  them  under  new  conditions,  until  some  congenital  variation  happens  to 
appear  of  a  proper  adaptive  character.  The  significance  of  this  conception  is 
perhaps  not  evident  at  a  glance.  It  may  be  made  clear  by  considering,  for 
illustration,  the  problem  of  the  development  of  habits  and  organs  adapted  to 
each  other.  .  .  . 

"  Perhaps  a  concrete  case  may  make  this  somewhat  obscure  theory  a  little 
clearer.  Imagine,  for  example,  that  some  change  in  conditions  forced  an  early 
monkey-like  animal  that  lived  on  the  ground  to  escape  from  its  enemies  by 
climbing  trees.  This  arboreal  habit  was  so  useful  to  him  that  he  continued 
it  during  his  life,  and  his  offspring,  being  from  birth  kept  in  the  trees,  acquired 
the  same  habit.  Now  it  would  be  sure  to  follow  that  the  new  method  of 
using  their  muscles  would  soon  adapt  them  more  closely  to  the  duty  of  climb- 
ing. Changes  in  the  development  of  different  parts  of  the  body  would  in- 
evitably occur  as  the  direct  result  of  the  new  environment,  and  they  would 
all  be  acquired  characters.  The  children  would  develop  the  same  muscles, 
tendons,  and  bones,  since  they,  too,  lived  in  the  trees  and  had  the  same  influ- 
ences acting  upon  them.  Such  acquired  characters  would  enable  the  ani- 
mals to  live  in  the  trees,  and  would  thus  determine  which  individuals  should 
survive  in  the  struggle  for  existence,  for  those  modified  individuals  would 
clearly  have  the  advantage  over  those  that  stayed  on  the  ground,  or  did  not 
become  properly  adapted  to  arboreal  life  by  acquired  habits.  All  this  would 
take  place  without  any  necessity  for  a  congenital  variation  or  the  inheritance 
of  any  character  which  especially  adapted  the  monkey  for  life  in  the  trees. 

"But  in  the  monkeys  thus  preserved,  congenital  variations  would  be  ever 
l  The  Method  of  Evolution,  p.  308  et  seq. 


I98  APPENDIX 

appearing  in  all  directions.  It  would  be  sure  to  follow  that  after  a  time  there 
might  be  some  congenital  variation  that  affected  the  shape  of  the  hands  and 
feet.  These  would  not  be  produced  as  the  result  of  the  use  of  the  organs  or 
as  acquired  variations,  but  simply  from  variations  in  the  germ  plasm.  There 
might  be  thousands  of  other  variations  in  other  parts  of  the  body  in  the  mean- 
time. The  miscellaneous  variations,  however,  would  not  persist.  But  as 
soon  as  variations  appeared  which  affected  the  shape  of  the  hands  and  feet, 
the  fact  that  the  animal  had  continued  to  climb  trees  would  make  these  varia- 
tions of  value,  and  therefore  subject  to  natural  selection.  Selection  would 
follow,  and  thus  in  time  the  monkeys  might  be  expected  to  inherit  hands  and 
feet  well  adapted  for  climbing.  The  acquired  variations,  in  such  a  case, 
had  nothing  to  do  with  producing  the  changes  directly,  but  they  did  shield 
the  animal  from  destruction  until  congenital  variations  appeared.  Acquired 
variations  have  determined  that  the  individuals  shall  live  in  trees,  and  this 
life  has  determined  what  congenital  variations  will  be  preserved.  Indirectly, 
therefore,  acquired  variations  guide  evolution." 

On  page  28  I  wrote:  "In  a  species  which  withstands  unfavorable  environ- 
mental conditions  through  the  plasticity  of  its  individual  members,  each 
individual  will  need  to  be  educated  into  harmony  with  the  environment. 
Such  individuals  of  the  species  as  vary  toward  greater  natural  adaptation  will 
need  less  education.  Of  course  innate  adaptation  is  more  advantageous  than 
adaptation  through  education,  since  it  is  immediate,  no  period  of  disadvan- 
tage appearing  in  the  early  life  of  the  individual.  The  death-rate  of  the  in- 
dividuals which  become  adapted  through  education  may  be  greater  than  that 
among  the  individuals  with  more  perfect  innate  adaptation.  Thus,  in  time, 
innate  adaptation  may  be  established  for  the  species  as  a  whole." 

When  the  innate  adaptation  is  by  means  of  a  character  similar  to  that  ac- 
quired by  the  plastic  individuals  through  education,  the  only  advantage  which 
the  innately  adapted  will  have  will  be  from  the  fact  that  they  pass  through 
no  stage  in  their  youth  when,  being  as  yet  insufficiently  modified,  they  are  not 
well  adapted  to  their  environment.  This  is  a  real  advantage  and,  in  a  species 
whose  individuals  become  modified  slowly  or  imperfectly,  the  advantage 
to  the  innately  adapted  may  be  of  selection  value.  But  if  the  ontogenetic 
adaptations  are  prompt  and  sufficient,  the  innately  adapted  individuals  will 
have  but  little  advantage.  That  is,  when  plasticity  is  very  marked,  it  will 


APPENDIX 


199 


greatly  lessen,  and  may  almost  remove,  natural  selection  so  far  as  these  par- 
ticular adaptive  qualities  are  concerned.  A  high  degree  of  plasticity  hinders 
the  development  of  innate  qualities  by  selection,  because  it  diminishes  the 
selection.  Plasticity  obstructs  selection.  As  an  example,  think  of  humankind. 
(Compare  page  177.) 

This  is  true  if  we  are  considering  innate  adaptations  of  the  same  sort  as 
those  produced  by  education.  But  it  is  not  so  true  if  we  consider  different 
types  of  modification  in  the  two  cases.  An  ontogenetic  modification,  such  as 
a  forced  change  in  habit,  leading  to  a  change  in  habitat,  may  enable  the  in- 
dividuals of  a  species  to  escape  destruction.  (Compare  Conn's  illustration 
of  the  acquired  arboreal  habit.)  Some  individuals  may  later  be  born  with 
an  innate  taste  for  tree-climbing,  but  this  would  hardly  give  them  great  ad- 
vantage over  the  other  individuals  which  took  to  the  trees  generation  after 
generation  because  they  had  to,  rather  than  because  they  wanted  to.  It  is 
doubtful  if  the  innate  instinct  to  climb  trees  would  be  promptly  established 
by  natural  selection  through  the  extermination  of  the  more  reluctant  tree- 
climbers.  The  forced  habit  of  tree-climbing  would  not  in  this  case  cause 
the  prompt  evolution  of  an  innate  tree-climbing  instinct. 

But  in  Conn's  illustration  of  the  acquired  arboreal  habit,  it  was  not  the 
instinct  of  tree-climbing,  but  foot  and  hand  structure  suitable  for  climbing, 
which  were  evolved.  The  arboreal  habit  adopted  by  the  several  individuals, 
generation  after  generation,  brought  the  animals  into  a  new  environment, 
and  here  new  structural  features  became  advantageous  and  were  evolved. 
The  ontogenetically  acquired  habit  did  not  cause  the  evolution  of  a  similar 
innate  habit,  but  caused  the  evolution  of  something  very  different,  namely, 
special  foot  and  hand  structure.  The  character  acquired  through  plasticity 
(tree-c'imbing  habit)  did  not  serve  as  a  close  guide  to  evolution,  but  as  a 
general  influence  toward  the  production  of  a  different  type  of  adaptation  to 
arboreal  life.  We  are  thus  led  to  the  conclusion  that  the  plastic  response  of  the 
individual  is  not  a  close  guide  to  the  course  of  evolution.  In  a  species  whose 
individuals  are  highly  plastic,  the  ontogenetic  modifications  will  usually  be  of 
a  different  sort  from  the  adaptive  innate  characters  which  may  arise  later. 

\Ye  come,  then,  to  this  general  result:  In  a  species  whose  members 
are  but  slightly  plastic,  or  slowly  responsive  to  modifying  influences,  innate 
characters  similar  to  those  ontogenetically  acquired  may  be  evolved;  but, 


200  APPENDIX 

in  a  species  whose  members  are  highly  plastic  and  rapidly  responsive,  the 
adaptive  innate  characters,  which  may  later  be  produced,  will  probably  be 
of  a  type  different  from  that  of  those  ontogenetically  acquired.  In  other 
words,  the  greater  the  plasticity,  the  less  intimate  will  be  its  guidance  of  the 
course  of  evolution,  for  a  rapidly  acquired  and  highly  developed  ontogenetic 
adaptation  is  almost  as  beneficial  as  an  innate  adaptation  of  the  same  type. 

Note  further  that  it  is  in  cases  of  change  in  the  environment,  or  change  in 
the  habitat  of  the  species,  that  the  chief  influence  of  plasticity  upon  evolution 
is  felt.  When  the  environment  remains  unchanged,  evolution  is  less  rapid 
and  the  influence  of  plasticity  is  also  less. 

Were  the  author  to  state  dogmatically  his  belief  as  to  the  role  of  plasticity 
in» evolution,  he  would  say:  The  accommodation  of  the  individual  to  adverse 
conditions  is  of  great  importance  in  enabling  the  species  to  survive  during 
a  period  of  temporary  disadvantage;  it  may  serve  in  a  general  way  to  guide 
the  course  of  evolution,  but  this  guidance  is  not  intimate  and  exact;  in  the 
case  of  species  whose  members  are  highly  plastic,  it  is  an  important  hindrance 
to  the  evolution  by  selection  of  qualities  of  similar  use  to  those  in  which  the 
plasticity  is  shown. 

Of  course  plasticity  is  itself  a  very  useful  quality  under  many  conditions 
and  will  be  developed  through  natural  selection. 


APPENDIX    II 

A    FEW    BOOKS    WHICH    TREAT    OF     ORGANIC    EVOLUTION 
AND    PHENOMENA    OF    SPECIAL    ADAPTATION 

DARWIN  :  The  Origin  of  Species.  Presents  the  theory  of  natural  selection 
with  a  wealth  of  description  of  phenomena  bearing  upon  it. 

The  Descen  0}  Man.     Treats  especially  sexual  selection. 

WALLACE  :  Darwinism.  Gives,  on  the  whole,  the  best  statement  of 
natural  selection  ;  treats  variation  well,  except  that,  of  course,  the  recently 
recognized  distinction  between  unstable  variation  and  mutation  is  not  men- 
tioned ;  is  interesting  in  its  criticism  of  sexual  selection ;  suggests  the  use  of 
colors  for  signals  and  recognition  marks ;  does  not  adequately  treat  segrega- 
tion ;  claims  that  natural  selection  is  insufficient  to  account  for  the  evolution 
of  the  hunmn  mind. 

Island  Life.  Gives  a  good  statement  of  the  phenomena  of  geographical 
distribution  in  their  bearing  upon  evolution. 

ROMANES:  Darwin  and  After  Darwin,  three  volumes.  Vol.  I,  Natural 
and  Sexual  Selection  and  the  natural  phenomena  which  bear  upon  them; 
very  clearly  stated,  many  good  illustrations.  Vol.  II,  Heredity  and  Utility: 
in  part  a  discussion  of  the  inheritance  of  parental  modifications.  Vol.  Ill, 
Isolation  and  Physiological  Selection:  the  best  statement  of  the  influence 
of  segregation  upon  evolution. 

WEISMANN  :  Essays  upon  Heredity  and  Kindred  Biological  Problems.  A 
very  valuable  and  stimulating  book  in  which  is  developed  the  theory  of  the 
continuity  of  the  germ  plasm  and  the  non-inheritance  of  parental  modifications. 

The  Germ  Plasm.  A  fuller  statement  of  Professor  Weismann's  theory 
of  the  continuity  of  the  germ  plasm :  somewhat  intricate. 

Germinal  Selection.     Supplementary  to  The  Germ  Plasm. 

The  Evolution  Theory.  Translated  by  J.  Arthur  Thompson.  A  sum- 
mary of  Professor  Weismann's  contributions  to  the  theory  of  evolution,  written 
for  general  readers  as  well  as  special  students. 


202  APPENDIX 

CONN  :  Tlie  Method  oj  Evolution.  A  readable  statement  of  the  theory, 
including  its  more  modern  phases,  except  that  mutation  is  not  treated. 

HUXLEY:  Man's  Place  in  Nature.  Giving  comparisons  between  man 
and  the  apes. 

Many  of  Huxley's  essays  deal  with  the  theory  of  evolution,  especially 
those  collected  in  the  two  volumes  Darwiniana  and  Evolution  and  Ethics. 

LLOYD  MORGAN:  Animal  Life  and  Intelligence  and  Animal  Behaviour. 
Morgan  is  a  very  discriminating  thinker  in  problems  of  heredity  and  evolu- 
tion, and  his  writings  are  very  helpful  as  well  as  very  readable. 

LUBBOCK:  The  Origin  0}  Civilization,  also  a  second  volume,  supple- 
mentary to  this,  entitled  Prehistoric  Times.  Very  interesting  volumes,  but 
by  many  regarded  as  unsound. 

WESTERMARCK:  The  History  0}  Human  Marriage.  Largely  a  reply  to 
Lubbock's  Origin  of  Civilization. 

T.  H.  MORGAN:  Evolution  and  Adaptation.  Contains  an  interesting 
criticism  of  the  theory  of  sexual  selection ;  gives  a  good  statement  of  the  theory 
of  mutation;  and  attempts  to  minimize  the  importance  of  natural  selection 
by  advocating  the  belief  that  evolution  may  occur  through  mutation  unaided 
by  natural  selection. 

There  are  many  books  upon  the  theory  of  evolution,  but  those  mentioned 
are  perhaps  as  important  as  any  for  one  who  is  not  familiar  with  the  subject. 
The  author  knows  of  no  satisfactory  presentation  of  evolution  from  the  stand- 
point of  those  who  believe  in  the  inheritance  of  parental  modifications.  COPE'S 
Origin  0}  the  Fittest  and  The  Factors  o)  Organic  Evolution  are  two  of  the  most 
important  books  written  from  this  standpoint,  but  they  are  very  difficult 
reading,  almost  unintelligible  in  parts.  LE  CONTE'S  Evolution  and  its  Rela- 
tion to  Religious  Thought  is  written  from  this  point  of  view,  but  it  is.  uncritical, 
assuming  rather  than  discussing  the  inheritance  of  parental  modifications. 

There  are  also  many  books  dealing  with  the  phenomena  of  adaptation, 
which  have  such  an  intimate  relation  to  the  theory  of  evolution.  COULTER'S 
Plant  Life  and  JORDAN  and  KELLOGG'S  Animal  Life  are  written  from  the 
point  of  view  of  evolution,  and  are  not  only  valuable  for  the  information  they 
convey,  but  are  very  readable  and  entertaining.  KERNER'S  Natural  History 
of  Plants,  translated  by  Oliver,  is  a  great  storehouse  of  information  as  to  special 
adaptations  seen  in  plants.  It  is  an  expensive,  four-volume  work,  but  should 


APPENDIX  20., 

be  found  in  all  libraries.  POULTON'S  The  Colours  of  Animals  gives  the  best 
treatment  of  this  interesting  subject.  GRANT  ALLEN'S  The  Colours  of  Flowers 
suggests  very  interesting  conceptions  as  to  the  evolution  of  the  colors  of  blos- 
soms. Its  contentions  are  not  fully  admitted  by  botanists,  but  it  is  well  worth 
reading. 

If  any  of  the  readers  of  this  Outline  are  interested  to  read  further  in  regard 
to  evolution,  the  author  would  suggest  that  ROMANES'  Darwin  and  After  Dar- 
win, Vols.  I  and  III,  and  WALLACE'S  Darwinism,  followed  by  WEISMAXN'S 
Essays  upon  Heredity,  would  probably  be  the  best  books  to  read  first,  and  with 
these  COULTER'S  Plant  Life  and  JORDAN  and  KELLOGG'S  Animal  Life. 

To  the  foregoing  list  may  be  added  the  following  titles  of  more  recent 
books : — 

DE  VRIES  :  Species  and  Varieties,  their  Origin  by  Mutation,  1905.  An 
elaborate  statement  of  the  theory  of  mutation  and  the  evidence  in  its  favor,  by 
its  author. 

Plant  Breeding,  1907.  Describes  the  origin  of  many  breeds  (species, 
incipient  species)  of  domestic  animals  and  plants  by  mutation,  and  mentions 
many  instances  of  the  repeated  origin  of  the  same  mutations,  i.e.  instances 
of  determinate  mutation.  A  readable  and  very  valuable  compilation  of 
originally  widely  scattered  data.  Probably  the  best  introduction  to  the 
phenomena  and  theory  of  mutation. 

JORDAN  AND  KELLOGG:  Evolution  and  Animal  Life,  1907.  Readable, 
well  illustrated,  good  literature  lists. 

KELLOGG:  Datwinism  To-dav,  1907.  A  good  critical  discussion  with 
very  good  literature  lists. 

THOMSON  :     Heredity,  1908.     Readable,  discriminating. 

BATESON  :  Mender s  Principles  of  Heredity,  1909.  Contains  a  translation 
of  Mendel's  original  papers  upon  hybridization. 

IJOCK.  :  Recent  Progress  in  the  Study  of  Variation,  Heredity,  and  Evolu- 
tion, 1910.  A  good  succinct  exposition  of  Mendelism  and  the  mutation 
theory. 

The  student  of  evolution  can  hardly  fail  to  be  interested  in  the  problems 
of  eugenics  as  related  to  mankind.  The  best  introduction  to  this  field  is  found 
in  KELLICOTT'S  The  Social  Direction  of  Human  Evolution,  an  outline  of  the 
science  of  Eugenics,  1911. 


INDEX 


(Italicized  page  numbers  and  plate  numbers  indicate  illustrations  of  the  subject  mentioned.) 


Abraxas  gloss  ularia,  PI.  70. 

Acquired  characters,  70. 

Acrcea  egina  and  gea,  PL  77. 

Acraida,  132,  137,  PL  76,  PL  77. 

Acronyda  alni  and  psi,  PI.  71. 

Adalm  bipunctata,  PL  69. 

Adaptation,  innate,  29;  not  explained  by 
the  theory  of  the  inheritance  of  parental 
modification,  81,  of  the  individual,  29, 
30,  177,  180,  196. 

^Egialitis  wcifera,  PL  82. 

Agassiz's  cave  fish,  97. 

Aggressi\-e  coloration  and  resemblance, 
127-129;  mimicry,  148. 

Alaska,  former  mild  climate  of,  65,  114. 

Alchemy,  Intro,  ix. 

Alga,  34,  38,  93,  «•  21. 

Algiers,  alluring  color  in  lizard,  131. 

Allen,  Grant  —  colors  of  flowers,  165,  202. 

Allen,  J.  A.  —  variation  in  Florida  birds,  9. 

Alluring  colors  and  resemblances,  129-131. 

Altruism,  27,  179,  183,  185. 

Amauris  echeria  and  ni-amus,  PL  76. 

Amazon  Valley  —  butterflies,  54 ;  leaf- 
cutting  ants  and  tree-hoppers,  140, 

PL  75- 

Amblystoma,  tadpole,  100. 

"American  Food  and  Game  Fishes" 
(Jordan  and  Evermann),  PI.  48,  PI.  58. 

Amoeba,  inheritance  of  parental  modifica- 
tions, 71 ;  reproduction,  71 ;  simple  or- 
ganization, 93. 

Anatomy,  comparative,  89,  90,  98. 

Ancon  sheep  —  segregation,  65,  a  sport,  41. 

''  Animal  Behaviour"  (Lloyd  Morgan),  202. 

"Animal  Life"  (Jordan  and  Kellogg), 
65,  97,  202,  203. 

"Animal  Life  and  Intelligence"  (Lloyd 
Morgan),  202. 

Antarctic  continent,  former  existence  of, 
116. 

Antelope,  confusing  coloration,  151,  PI.  81; 
protective  color,  122;  signalling,  149, 
PL  81. 


Antlers  — of  deer,  108,  109,  PL  43;  of 
elk  —  correlation  with  ligamentum  nu- 
chtz,  37  — over  development,  50. 

Ants  —  antlike  spiders,  140,  141,  148; 
mimicked  by  tree-hoppers,  140,  PI.  75; 
parasites,  149;  unpalatable,  140. 

Apatura  iris,  PL  56. 

Ape  — related  to  Hominida,  166;  ear 
of  Barbary  ape,  PL  94;  nictitating 
membrane,  PL  36. 

Apis  mellifera,  PL  74. 

Aplecta  occulta,  PL  55. 

Apocyrtus,  PL  73. 

Appendix,  vermiform,  man  and  orang,  169, 
PI.  96. 

Apteryx,  vestigial  wings,  96,  PL  35. 

Arbutus,  trailing,  7,  157. 

Arch&opteryx  lithographica,  in,  PL  44. 

Arctia  caja,  PL  70,  PL  71. 

Arctic  fox,  1 28,  129. 

Argus  pheasant,  PL  24. 

Ariamnes  attenuate,  PL  64. 

Aristolochia  sipho,  161,  PL  90. 

Artificial  selection,  30-33. 

Asexual  reproduction  and  inheritance  of 
parental  modifications,  74. 

Astia  vittata,  var.  nigra,  PL  28. 

"Astrolabe,  Voyage  de  1',"  148. 

Altacus  atlas,  146. 

Attractiveness,  criteria  in  choice  in  mar- 
riage, 184,  185,  186. 

Australasia  —  fauna,  115;  limits  of  fauna 
(map),  117. 

Avebury,  Lord,  color  sense  in  insects, 
162  et  seq.,  202. 

Bacteria,  rate  of  increase,  14. 

Bagvvorm,  23. 

Baldwin,  J.  Mark,  29,  192. 

Bali  —  Lombok  strait  (map),  117. 

Barnacle  goose,  3,  5;  goose  barnacle,  3, 

4,  5;  sacculina,  187,  188,  189,  PL  101. 
Barriers  to  migration,  114. 
Basilarcha  (Limenitis)  disippus,  141,  PL  76. 


205 


200 


INDEX 


Bastin,  E.  S.,  PI.  88. 

Bat  —  skeleton  of  wing,  94. 

Bates,  H.  W.  —  butterflies  of  Amazon 
Valley,  54;  terrifying  attitude  in  cater- 
pillar, 142,  143- 

Bateson,  203. 

Bear,  polar,  128. 

Beddard,  F.  E.,  124,  129,  PI.  62. 

Bee,  carrying  pollen,  157;  color  prefer- 
ence, 162,  165;  evolution  of  instincts, 
79;  honey-bee,  three  types,  24;  mim- 
icked by  flies,  PI.  74;  mimicked  by 
moths,  PI.  70;  parasites,  149;  pro- 
tected by  stings,  132,  137;  sacrifice 
of  individuals  for  benefit  of  hive,  24  ; 
sterility  of  workers,  25 ;  warning  color, 

132,  PL  74- 

Beetle,  Colorado  potato-beetle,  133,  PL  69; 
color  preference,  165;  curculio,  136, 
PI-  73!  goldenrod-beetle,  133;  Her- 
cules beetle,  54,  PL  30;  lady-beetle, 

133,  U7,  PI-  69,  PL  73;    leaf  beetle, 
125,  PL  62;  mimicry,  138,  140,  PI.  73; 
sexual    divergence,    54,    55;     staghorn 
beetle,  54,  PL  29;  warning  color,  133. 

Behring  Straits,  migration  across,  65, 
114. 

Belt,  moss  insect,  124,  PI.  61. 

Biology,  Intro,  xxi,  xxii. 

Bird,  confusing  coloration,  151;  females 
protectively  colored,  53 ;  gastrula,  PI. 
42;  males  serve  as  decoys,  53;  mim- 
icry, 147,  PL  80;  noxious  insects, 
132;  protective  color,  120,  PI.  49-51; 
recognition  marks,  150,  PI.  82;  segre- 
gation, 68;  sexual  coloration,  152,  153; 
sexual  phenomena,  52-53;  sexual  selec- 
tion, 56;  skeleton,  94,  112. 

"Birds  of  Eastern  North  America,  Hand- 
book of"  (Chapman),  52. 

"Birds  of  New  Guinea"  (Gould),  PI.  25, 
PI.  26. 

Birth-rate,  13,  14;  relation  to  struggle  for 
existence,  20. 

Blackbird,  sexual  coloration,  153. 

Blastomere,  73. 

Blastopore,  103,  104,  105,  PL  42. 

Blind  fish,  97. 

Blossoms,  see  Flowers. 

Bluebird,  sexual  coloration,  153. 

Blue  crab,  PI.  40. 

Bluefish,  120,  PL  48. 

Boa  constrictor,  vestigial  limbs,  96. 


Boar,  correlation  between  tusks  and  bristles, 

38. 

Bobolink,  sexual  coloration,  153,  PL  22. 

Bolton,  Gambier,  PI.  68. 

Bombus  -Vancouver ensis,  PL  74. 

Bonasa  umbellus,  PL  23. 

Borecole,  PL  6. 

Bourru,  friar-bird  and  oriole,  147,  148. 

Brassica  oleracea,  Pis.  4,  a,—8;  rapus,  PL  9. 

Breeding,  in  and  in,  44;  methods  used,  31 ; 
mice  (Castle),  46;  breeding-time  and 
segregation,  45. 

Brehm,  4,  24,  33,  135,  PI.  24,  PI.  30,  PI.  33, 
PL  62,  PI.  99. 

Britcher,  H.  W.,  140,  PL  2,  PL  64,  PL  85. 

Britton,  N.  L.,  90,  91,  92. 

Broccoli,  PL  7,  PL  8. 

Bronze,  invention  of,  179. 

Brown,  Addison,  90,  91,  92. 

Brussels  sprouts,  PL  7. 

Bufo  lentiginosus,  PL  66. 

Bugs,  noxious  character  and  warning 
color,  133,  PL  69. 

Butomus  umbellatus,  PL  86. 

Butterfly,  color  preference,  165;  confusing 
coloration,  150,  PL  83;  courting,  58; 
leaf  butterflies,  125,  PL  83;  mimicry, 
140,  141,  PL  76,  PL  77;  sexual  color- 
ation, 54,  PL  84;  warning  color,  133, 
PL  59,  PL  76,  PL  77,  PL  84. 

Cabbage,  varieties  of,  31,  Pis.  4,  a  -8. 

Calamesia  midama,  141,  153,  PL  84. 

Calamus  arctifrons,  PL  58. 

Calf  embryo,  PL  38. 

Callimorpha  dominula  and  hera,  PL  70. 

Callinectes  hastatus,  PL  40. 

Callionymiis  lyra,  PL  32. 

Calocalanus  plumulosus  and  paw,  57. 

Calopteryx  maculala,  PL  33. 

Cambrian  fossils,  108. 

Cambridge,  PL  64. 

"Camera  Shots  at  Big  Game"  (Wallihan), 
PL  81. 

Cancer  pagurus,  102. 

Capital  punishment,  27. 

Carboniferous  fossils,  108. 

Cardinal,  sexual  coloration,  153. 

Carrion  flower,  157. 

Castle,  W.  E.,  mendelian  phenomena,  46. 

Caterpillars,  protective  color,  PL  56;  terri- 
fying attitude,  142, 143,  PL  78;  warning 
color,  PL  71. 


INDEX 


207 


Catocala  amatrix,  PI.  60;  concubens,  PI.  83. 

Cauliflower,  PI.  7,  PI.  8. 

Cave-dwelling  animals,  eyes,  95. 

Cemop flora  coccinea,  PL  79. 

Ceram,  friar-bird  and  oriole,  147. 

Ccratophora  stoddartii,  PI.  34. 

Cerebral  hemispheres,  man,  167,  PI.  92. 

Cerura  vinula,  143,  PL  78. 

CervKS,  antlers,  109,  PL  43. 

Charocampa  elpenor,  142, 143,  PL  78. 

Chanteleo  bifurcus  and  owenii,  PL  34. 

Change,  in  environment  makes  evolution 
rapid,  28;  of  function  in  organs,  40. 

Chapman,  Frank  M.,  52. 

Chaidiodes  cornntus,  PL  31. 

Chemistry,  Intro,  xxi. 

Chemistry,  relation  of  life  processes  to,  191. 

Chickens,  breeds  of,  31,  32,  Pis.  12-19; 
embryos,  PL  38;  reject  noxious  insects, 
132;  sexual  coloration,  153,  Pis.  12-15; 
variations  in  fecundity  of  unstable  type, 
10. 

Chimpanzee,  167,  Pis.  91-94. 

Chincha,  various  species,  PL  72. 

Chinese,  segregation,  171. 

Choice  in  mating,  51,  in  marriage,  183. 
See  Sexual  selection. 

Chologaster,  97. 

Chordeiles  virginianus,  PL  82. 

Christianity.,  effect  on  evolution,  182. 

Chromatin,  controls  inheritance,  72. 

Cicada  septetndecim,  54,  PL  29. 

Cidaria  cucullata,  PL  56;  galiata  and  occl- 
lala,  PL  55. 

Classification,  90. 

Claus,  C.,  PI.  41. 

Climate,  cause  of  segregation,  64;  of 
Alaska  and  Siberia,  55. 

Cobra,  imitated  by  moth,  145,  146. 

Cocdnella,  PL  73. 

Coccyx,  man  and  apes,  167,  168,  PL  91. 

Cccrostris  mitralis,  PL  64. 

Colewort,  wild,  PL  4,  a;  varieties  of,  Pis. 
5-8- 

Colinus,  virginianus,  PL  49. 

"College  Botany"  (Bastin),  PI.  88. 

Coloborhombits  fasciatipennis,  PL  73. 

Color,  adaptation  in  butterfly  pupae,  123, 
PL  59;  aggressive,  127-129;  alluring, 
129-131;  animals,  118-154;  change, 
123,  PL  58;  classification  of  color  phe- 
nomena, 118;  confusing,  150-152;  con- 
vergence in  warning  color,  136 ;  flowers, 


154-166;  immunity,  118,  137,  154; 
insects  and  color  of  flowers,  162  et  seq.; 
mimicry,  137-149;  recognition  marks, 
149-150;  seasonal,  123, 128, 129,  PL  57; 
sexual,  55,  152-154;  signals,  149-150; 
use  and  disuse  and  color  of  flowers,  78 ; 
warning,  131-136. 

Colorado  potato-beetle,  133,  PL  69. 

"Colors  of  Animals,  The"  (Poulton), 
118,  142,  202. 

"Colors  of  Flowers,  The"  (Grant  Allen), 
165,  166,  202. 

Columbia  lima,  33,  PL  20. 

Community,  length  of  life  in  communal 
animals,  23 ;  unit  in  struggle  for  exist- 
ence, 26. 

Complexity,  increases  during  growth  of 
an  organism,  98;  in  more  recent  fos- 
sils, 1 08;  varying  degrees  of,  92,  99. 

Composites,  92. 

Confusing  coloration,  150-152,  PL  83. 

Conn,  H.  W.,  196,  202. 

Convergence  in  warning  coloration,  136. 

Cony,  protective  color,  PL  54. 

Cope,  E.  D.,  PI.  46,  202. 

Copepoda,  60. 

Coral  polyp,  gastrula,  104. 

Correlation  of  organs,  35,  36,  40;  between 
innate  characters  and  attainments,  180; 
between  vigor  aiid  secondary  sexual 
characters,  61. 

Corvits  americanus,  112. 

Cotton-tail  rabbit,  confusing  coloration, 
151 ;  protective  coloration,  PL  53;  sig- 
nals, 149. 

Coulter,  J.  M.,  202,  203. 

Courtship,  birds,  51,  PL  23,  PL  24,  PL  27; 
fish,  54,  153;  Groos  on  relation  of 
courtship  to  natural  selection,  56;  not 
observed  in  some  forms  which  show 
divergence  in  secondary  sexual  charac- 
ters, 58 ;  observation  difficult,  61  ; 
spiders,  52,  PL  28. 

Crab,  101,  102;  blue  crab,  PL  40;  protec- 
tive color,  122  ;  sacculina  parasitic  upon 
crab,  1 88,  PL  101 ;  which  resembles  a 
pebble,  126,  128. 

Crawfish,  nervous  system,  PL  40;  protec- 
tive color,  122. 

Creation,  Theory  of  Special,  Intro,  xxi. 

Cross-breeding  swamps  varieties,  43. 

Cross-fertilization,  44,  156-161,  Pis.  88-90. 

Crustacea,    development    of    higher,    100, 


208 


INDEX 


Pis.    39-41;    pelagic   Crustacea   trans 
parent,  119;   protective  color,  122. 

Cryptolithodes  silchensis,  126. 

Ctenophores,  transparency  of,  119. 

Curculio,  135,  PI.  73. 

Curlew,  protective  color,  121. 

Cydoptera,  PI.  62. 

Cypris,  188. 

Dahlia,  varieties  of,  31,  PI.  10,  PI.  u. 
Daisy,  marguerite  —  rate  of  increase,  15. 
Danaida,  134,  140,  PI.  76,  PI.  84. 
Danais  archippus,  140,  PI.  76;  chrysippus, 

PI.  76. 
Darwin,  Charles,  50,  51,  55,  201,  PI.  32, 

PI-  34,  PL  95- 

"Darwin  and   after  Darwin"  (Romanes), 
107,  201 ,  203,  PI.  1 7,  PI.  36,  PI.  37,  PI.  44, 
PI.  75,  Pis.  93-97,  PL  99- 
"Darwiniana"  (Huxley),  202. 
"Darwinism"    (Wallace),    37,    134,    147, 

201,  203. 

"Darwinism  To-day"  (Kellogg),  203. 
Dasychira  piidibunda,  PL  55. 
Datana  mmislra,  PI.  75. 
Dean,  Forest  of,  segregation  of  deer  in,  68. 
Death,  at  close  of  reproductive  period,  22, 
24,  25,  26;  bagworm  moth,  23;  capital 
punishment,    27;     causes,    16;     drone 
bees  and   young   queen   bees,    24,    25; 
of  individual  for  general  welfare,  22-27  ; 
rate,  14. 

Dccapoda,  development  of,  100,  Pis.  39-41. 
Decoy,  male  birds  serve  as,  53. 
Deer,  antlers,  108,  109,  PI.  43;  protective 
color,    122;    secondary   sexual   charac- 
ters, 56;   segregation,  68;   signals,  149. 
Degeneration,  187-190. 
Delage,  Yves,  PI.  101,  189. 
"Descent  of  Man"  (Darwin),  55,  201. 
Deterioration  due  to  escape  from  struggle 
for  existence,  173;    due  to  parasitism, 
1 88  el  scg. 

Devonian  fossils,  108. 
De  Vries,  Hugo  —  mutation,  10,  20,  21,  41, 

193,  194,  195,  203. 
DianthcEcia  compta,  PL  55. 
Diapheromera  femorata,  PL  61. 
Diatoms,  skeletons  of,  33,  PL  21. 
Dimorphism,  in  flowers  of  Milchella,  159, 

PL  88;  sexual,  see  Sexual  selection. 
Discontinuous  variation,  21. 
Disease,  172,  174. 


Dismal  Swamp  fish,  97. 
Dismorphia  astynome,  PL  77. 
Dissosteira  Carolina,  PL  83. 
Distribution,  geographical,  89,  113-118. 
Divergence,  10;   degree  of,  in  variation,  9; 
from    species    type,    an    advantage    in 
struggle  for  existence,   28;    in  relation 
to   mendelian   phenomena,   49;   sexual, 
see  Sexual  selection  ;  swamped  by  cross- 
breeding, 43. 

Dog,  mating,  51 ;   skeleton  of  lore  limb,  94. 
Dolichonyx  oryzivorus,  PL  22. 
Doliops  sp.  and  D.  curadionides,  PL  73. 
Doryphora  decemlineata,  PL  69. 
Dragon-fly,  PL  j/,  PL  33. 
Drone-fly,  imitates  bee,  138,  PL  74. 
Drouth  as  cause  of  segregation,  45. 
Dugmore,  A.  R.,  PI.  51,  PI.  50,  PI.  58. 
Dugong,  nictitating  membrane,  PL  36. 
Dynastes  hercules,  53,  PL  30. 

Eagle  —  nictitating  membrane,  PL  36. 

Ear,  change  of  function,  41 ;  man  and  apes, 
168,  Pis.  94-96;  vestigial  muscles  of 
human,  96,  97. 

Education  (plasticity),  29,  196;  suscepti- 
bility to,  hinders  Evolution,  180. 

Egypt,  178. 

Eimer,  orthogenesis,  49. 

Elaps,  145,  PL  79. 

Elephant  hawk  moth,  141,  142,  PL  78. 

Elk  —  correlation  between  antlers  and 
ligamentum  nuchce,  37;  Irish,  ortho- 
genesis, 50. 

Elymnias  phegea,  PL  77. 

Embryology,  comparative,  89,  98-105 ; 
of  higher  Crustacea,  91,  Pis.  39-41;  of 
man,  169,  PL  98;  of  vertebrates,  99, 
PL  38,  PI.  98. 

Emydia  jacobcte,  PL  70. 

environment,  and  nature  of  organism,  191  ; 
change  in  —  makes  evolution  rapid,  28; 
inheritance  of  direct  effects  of,  75; 
nature  of  man's,  17^. 

Zpeira  prompta  and  stellata,  PI.  64. 

^.pilobium  hirsutum,  142. 

Epipactys  latifolia,  PL  89. 

iqnus,  PL  47. 

trio  pus  purpureofasciata,  PL  55. 

Iristalis  tenax,  PL  74. 

2rrera,  PI.  4,  a. 

Erythrolamprus  esculapii  and  venustissi- 
mns,  PI.  7Q. 


INDEX 


209 


"  Essays  upon  Heredity  and  Kindred  Bio- 
logical Problems"  (Weismann),  201, 
203. 

Euchistus  servus,  PL  69. 

Eugenics,  xi ;   Kellicott,  186,  203. 

Euplaa  midamus,  141,  153,  PL  84. 

Evermann,  Barton  G.,  PI.  48,  PI.  58. 

"Evolution   and    Adaptation"    (Morgan), 

202. 

"Evolution  and  Animal  Life"  (Jordan  and 

Kellogg),  203. 

"Evolution  and  Ethics"  (Huxley),  202. 
"Evolution  and  its  Relation  to  Religious 

Thought"  (Le  Conte),  202. 
"Evolution,    The    Factors    of    Organic" 

(Cope),  202. 
"Evolution,  The  Method  of"  (Conn),  196, 

202. 

"Evolution  Theory,  The"  (Weismann), 
195,  201. 

Extirpation  of  organs,  effects  of,  36. 

Eye,  deterioration  of  human,  173  ;  of  cave- 
dwelling  animals,  97;  of  various  ver- 
tebrates, PL  97. 

Factors  in  evolution,  Intro,  xxiii,  85,  191. 

"Factors  of  Organic  Evolution,  The" 
(Cope),  202. 

Family,  26,  185. 

Faroe  Islands,  segregation  of  sheep,  68. 

Faunas  and  floras,  114,  115. 

Felis  tigris  and  onca,  PL  68. 

Fertilization,  cause  of  variation,  84 ;  cross 
and  self,  44;  of  flowering  plants,  155- 
166;  by  wind,  156;  by  insects,  isjelseq. 

Fescue-grass,  156,  PL  87. 

Field  sparrow,  PL  49,  1 28. 

Fish,  birth-rate,  14;  blind  fish,  97;  blue- 
fish,  120,  PL  48;  embryos,  PL  38; 
flatfish,  120,  PL  48;  immunity  colora- 
tion in  coral-reef  fishes,  137;  protec- 
tive coloration,  120,  PL  48;  sexual 
coloration,  55,  153;  sexual  divergence, 
PL  32. 

Flatfish,  120,  PL  48. 

Flies,  drone-fly,  mimics  bee,  138,  PL  74; 
fertilization  of  Aristolochia,  161;  fer- 
tilization of  white  flowers,  165;  mim- 
icry of  bees  and  wasps,  137,  PL  74- 

Flora,  114,  115- 

"Florida,  On  the  Mammals  and  Birds  of 
East  "(Allen,J.  A.),  9. 

Flounder,  protective  color,  PL  48. 
p 


Flower,  W.  H.,  96,  PI.  44,  PI.  68. 

Flowers,  diagrams  of  various,  PL  86; 
insect  visitors,  165;  landing  place  for 
insects,  166,  PL  89,  PL  90;  of  Aris- 
tolochia, 161,  PL  go;  of  Mitchdla,  159, 
PL  88;  of  orchid,  159,  PL  8g,  PL  90; 
of  Salma,  160,  PL  89;  of.  wind-fer- 
tilized plants,  156. 

Forbes,  H.  O.,  129,  147,  148. 

Fossils,  conditions  for  the  formation  and 
preservation  of,  106;  table  of  fos- 
siliferous  rocks,  107;  marsupials  of 
America,  115. 

Fowl,  see  Chickens. 

Fox,  aggressive  coloration.  128;  Arctic,  128, 
129;  segregation,  63. 

Fox,  Rev.  W.  D.,  mating  of  Chinese  geese, 
51- 

Friar-bird,  147,  PL  80. 

Frog,  aggressive  color,  128 ;  gastrula,  PL  42; 
noxious  insects,  132;  protective  color, 
122. 

Gagea  lutea,  PL  86. 

Galapagos  Islands,  segregation  of  locusts, 
63,  65- 

Galens,  nictitating  membrane,  PL  36. 

Galileo,  Intro,  xxi. 

Game  cock,  evolution  of,  PL  16. 

Gastrula,  coral  polyp,  104;  various  ani- 
mals, 105;  vertebrates,  104,  PL  42. 

Gazelle,  white  rump  patch,  149. 

General  considerations,  186-191. 

General  principles  in  operation  of  natural 
selection,  22. 

Geographical  distribution,  89,  113-118. 

Gerarde,  5. 

Germ  cells,  72;  and  variation,  82;  nutri- 
tion of,  83  ;  organization  of,  195. 

Germinal  selection,  98,  195. 

"Germinal    Selection"    (Weismann),    195, 

2OI. 

"  Germ  Plasm,  The"  (Weismann),  195,  201. 

Gibbon,  167,  PL  91. 

Giesbrecht,  60. 

Gila  monster  (lizard),  135,  PL  72. 

Giraldus,  Sylvester,  3. 

Goat,  protective  color  in  wild,  122. 

Goldfinch,    American,    sexual    coloration, 

153- 

Goodale,  William,  PI.  88. 
Goose,  barnacle,  3,  4,  5;   mating  of  white 

and   Chinese,   51 ;    variation   slight,  8. 


210 


INDEX 


Gorilla,  167,  PL  QI,  PI.  92. 

Gould,  PI.  25,  PL  80. 

Government,  progress  in,  1 79. 

Crackle,  sexual  coloration,  153. 

Grapta,  PL  83. 

Grass,  fertilization,  156. 

Grasshopper,  confusing  coloration,  150, 
PL  83;  leaf,  125,  PL  62;  mimicking 
beetles,  137,  PL  73. 

Grass  porgy,  123,  PL  58. 

Gravitation,  Intro,  xxi. 

Gray  snapper,  137. 

Gray's  "Anatomy,"  97,  PI.  96. 

Greeks,  179;  conception  of  origin  of  ani- 
mals from  plants,  3,  6. 

Greenland  whale,  skeleton,  96. 

Grip,  strength  of,  of  human  infant,  169, 
PL  wo. 

Groos,  courtship,  57,  60. 

Grouse,  birth-rate,  14;  ruffed  grouse,  120, 
PL  23;  snow  grouse,  123,  PL  57. 

Gulick,  John  T.,  segregation  of  land  snails 
of  Oahu,  66,  67. 

Habrocestum  howardii,  PI.  28;  splendens, 
PL  85. 

Haeckel,  Ernst,  104,  PI.  21,  PI.  38,  PI.  98. 

Hair,  change  of  function,  40;  of  man  and 
ape,  96,  167 ;  PL  37,  PL  93. 

"Handbook  of  Birds  of  Eastern  North 
America"  (Chapman),  52. 

Hapsburg  lip,  a  mutation,  182. 

Haswell,  W.  A.,  PI.  44. 

Hawaiian  Islands,  segregation  in  land 
snails,  66. 

Hawk  moth,  elephant,  142,  143,  PL  78. 

Hayes,  PI.  4. 

Heart,  change  of  function,  40. 

Hebomoia  glaucippe,  PL  83. 

Helianthemum  marifolium,  155. 

Heliconidae,  134,  PL  77. 

Heliconius  eucrate,  PL  77. 

Hdoderma  horridum,  PL  72. 

Hemiptera,  warning  color  and  shape,  133, 
PL  69. 

"Herball,The"(Gerarde),  5. 

Hercules  beetle,  54,  PL  30. 

Heredity,  3,  10,  20;  inheritance  of  paren- 
tal modifications,  70,  71,  178. 

"Heredity"  (Thomson),  203. 

"Heredity  and  Kindred  Biological  Prob- 
lems, Essays  upon"  (Weismann),  107, 
203. 


"Heredity  and  Utility"  (Romanes),  201. 

Herrick,  F.  H.,  PI.  43- 

Hes peridot,  130. 

Hcsperornis  regalis,  HI,  PI.  45. 

Hestia,  129. 

Heterocampa  biundata,  PL  56. 

Heterocampa  pulverea,  PL  55. 

Hippiscus  tuberculatus,  PL  83. 

Hippocampus  mohnikei,  126. 

Hippodamia  convergent,  PL  69. 

"History  of  Human  Marriage,  The"  (Wes- 
termarck),  202. 

Hog,  embryos,  PL  38. 

Homarus  amcricanus,  PL  39,  PL  41. 

Homer,  179. 

Hominidce,  167. 

Homology,  94,  95,  103. 

Homoptcra,  —  mimicking  ants,  140,  PL  75; 
edusa,  PL  54. 

Honey-bee,  24,  25. 

Honeysucker,  imitated  by  oriole,  146,  PL 
80. 

Hornet,  132,  137,  PL  74. 

Horse,  breeds  of,  30,  PL  4;  correlation  be- 
tween hair  and  hoofs,  38;  evolution  of 
feet  and  teeth,  in,  PL  46,  PL  47; 
gradual  change  in  horse  family,  42,  193, 
195  ;  nictitating  membrane,  PL  36;  seg- 
regation in  Paraguay,  68. 

"  Horse,  Points  of  the"  (Hayes),  PI.  4. 

Human  evolution,  166-186;  how  controlled, 
182,  186;  sexual  selection,  171,  172,  175, 
176, 178, 181, 182-186. 

Humming-bird,  —  nest,  PL  51 ;  sexual 
coloration,  153,  PL  26. 

Huxley,  T.  H.,  102,  202,  PI.  40,  PI.  91,  PI.  92. 

Hydra,  103. 

Hyla  vcrsicolor,  PL  66. 

Hymenoptera,  mimicked  by  other  insects 
and  spiders,  138,  139,  140,  141 ;  pecul- 
iar form,  133  ;  protected  by  stings,  132, 
139;  warning  color,  132,  PL  73,  PL  74. 

Hymenopus  bicornis,  130. 

Ichthyornis  victor,  in,  PL  45. 
Ichthyura  inclusa,  var.  invcrsa,  PL  55. 
Icius  mitratiis,  PL  28. 
Immortality,  183. 

Immunity  coloration,  118,  137,  154. 
Improvement  of  human  race,  1 76. 
In-breeding,  44. 

Incipient  species,  20;  not  infertile  when 
crossed,  34. 


INDEX 


211 


Indians,  North  American,  measles,  174. 

Indigo-bird,  sexual  coloration,  153. 

Individual  sacrificed  for  welfare  of  species, 
22,  190. 

Infant,  human  —  foot  position,  169,  PI.  100; 
spinal  curve,  169,  PL  gg;  strength  of 
grip,  169,  PL  100. 

Infertility,  domestic  races  not  infertile 
when  crossed,  33;  of  crosses  between 
species,  33, 34;  of  hybrids,  33  ;  starting- 
point  in  evolution,  34. 

Inherent  tendency,  in  variation,  42;  in 
evolution,  49-50,  86,  191,  193. 

Inheritance  of  parental  modifications, 
Intro,  xxiii,  67,  178. 

Injury  —  effects  of  —  inherited  among  uni- 
cellular organisms,  72. 

Innate  adaptation  vs.  acquired  adaptation, 
29 ;  character  vs.  training,  1 29  et  seq. 

I  no  primi  and  slat  ices,  PL  70. 

Insects,  and  color  of  flowers,  154  et  seq.; 
and  plant  fertilization,  157  et  seq.; 
protective  color,  122. 

Instinct,  41. 

Internal  factors  in  evolution,  191. 

Invalidism,  173,  176,  185. 

Inventive  genius,  1 79. 

''Ireland,  Relations  concerning"  (Giral- 
dus),  3. 

Island  faunas  and  floras,  114. 

"Island  Life  "(Wallace),  201. 

Isolation,  see  Segregation. 

"  Isolation  and  Physiological  Selection " 
(Romanes),  201. 

Jack-rabbit,  confusing  coloration,  151. 

Jaguar  —  aggressive  coloration,  1 29,  PL  68. 

Jamaica  —  Neritina,  g. 

Java  —  spider  which  resembles  bird  excre- 
ment, 129. 

Jellyfish,  mouth,  103;  transparency,  119. 

Jenner,  174. 

Jesus,  influence  upon  evolution,  182. 

Job,  179. 

Jordan,  D.  S.,  65,  97,  126,  202,  203,  PI.  48, 
PL  58. 

Jordan  and  Kellogg,  203. 

Juncus  bufonius,  seedpods  imitated  by  a 
spider,  126,  PL  64. 

Jungle-fowl,  31,  121,  PL  16. 

Junonia,  PL  83. 

Kale,  PL  6,  PL  8. 


Kallima  inachis,  125,  150,  PL  83. 
Kangaroo  rat,  confusing  coloration,  151. 
Kappel  and  Kirby,  PL  56,  PL  70,  PL  ji 

PL  76,  PL  77,  PL  78. 
Kellicott,  eugenics,  186,  203. 
Kellogg,  V.  L.,  65,  97,  202,  203. 
Kepler,  Intro,  xxi. 

Kerner,  A.,  155,  161,  202,  PL  86,  PI.  89. 
Killdeer,  recognition  marks,  150,  PL  82. 
Kirby,  see  Kappel. 
Kohlrabi,  PL  7,  PL  8. 

Lady-beetle,  noxious  character  and  warn- 
ing coloration,  133,  137,  PL  69,  PL 
73- 

Lagopus  lencurus,  PL  57. 

Landing-place  for  insects  in  plant  blos- 
soms, 1 66,  PL  89,  PL  go. 

Lang,  Arnold,  PL  41. 

Larvae,  transparency  of  marine,  1 20. 

Laurent,  PL  4,  a. 

Leaf-cutting  ants,  mimicked  by  tree-hop- 
pers, 140,  PL  75. 

Leaf-like  insects,  124,  125,  PL  56,  PL  62, 
PL  83. 

Le  Conte,  Joseph,  202. 

Lemur,  166. 

Lepas  analifera,  4. 

Lencania  l-albnm,  PL  55. 

Leucoma  solids,  PL  71. 

Lever,  invention  of,  179. 

Life,  length  of,  22,  25,  26;  processes,  chem- 
istry and  physics,  191. 

Limenilis  (Basilarcha),  disippus,  141,  PI. 
76;  sibylla,  PL  56,  PL  76;  populi, 
PI.  78. 

Lion,  aggressive  color,  128;  secondary 
sexual  characters,  56. 

Lizard,  aggressive  coloration,  128,  PL  52; 
alluring  coloration,  131 ;  confusing 
coloration,  152;  gila,  135,  PL  72; 
protective  coloration,  122,  PL  52; 
rejects  noxious  insects,  132;  sexual  col- 
oration, 152;  sexual  divergence,  PL  34. 

Lobster,  100,  122,  PL  39-41. 

Lock,  203. 

Locust  —  Galapagos  Islands,  65;  leaf, 
124,  PL  62. 

Logoa,  125,  PL  63. 

Lophornis  adorabilis,  PL  26. 

Love,  foundation  in  marriage,  183. 

Low,  Professor,  mating  of  domestic  ani- 
mals, 51. 


212 


INDEX 


Lubbock,  John,  see  Lord  Avebury. 
Lucanus  datna,  PI.  2Q. 
Lungs,  change  of  function,  41. 
Lycorea  halii,  PL  77. 
Lydekker,  Richard,  PI.  68,  PI.  72. 
Lymnaus,  development  of,  QQ. 
Lyre  bird,  PI.  24. 

McCook,  H.  C.,  PI.  75. 

MacDougal,  193. 

Macroglossa  bombyliformis  and  stellatarum, 

PL  70. 
Mammalia,  development,  gg,  PL  38;  fossils, 

107,  108. 
"Mammals  and    Winter    Birds    of    East 

Florida,  On  the"  (Allen),  9. 
Mammoth,  50. 

Mammoth  Cave,  blind  animals  in,  97. 
Man,  embryos,  PL  38;  evolution,  166-186; 

nictitating    membrane,    PL    36;     one 

species,  1 66;  plasticity,  30,  1 80;  sexual 

selection,  171,  172,  175,  176,  178,  180, 

182-186;    skeleton  of  arm,   95;    slow 

evolution,     178;      social    progress    vs. 

evolution,  176. 

"Man's  Place  in  Nature"  (Huxley),  202. 
Mantis,  alluring  color  and  form,  129,  130; 

leaf  mantis,  125,  128,  PL  62. 
Marguerite,  daisy,  rate  of  increase,  15. 
Marptusa  familiaris,  PL  28. 
Marriage,    human,    171;     choice    in,    see 

Man,    Sexual    selection;     laws,     184; 

responsibility  in,  184. 
"Marriage,     The     History     of     Human" 

(Westermarck),  202. 
Marsh,  O.  C.,  PI.  45,  PI.  47. 
Marshall,  A.  M.,  PI.  42. 
Marsupialia,     geographical      distribution, 

"5- 

Mastodon,  50. 

Mating,  preferential,  see  Sexual  selection. 
Mean,  species,  21. 
Measles,  among  savage  races,  174. 
Mechanitis  lysimnia,  PL  77. 
Megi/la  maculata,  PL  6g. 
Melintza  ethra,  PL  77. 
Melitaa  cinxia,  PI.  71. 
Mendel,  42,  46,  203. 

"Mendel's  Principles  of  Heredity"  (Bate- 
son),  203. 

Menura  siiperba,  PL  24. 
Mephitis  mephitica,  PL  72. 
Merriam,  C.  Hart,  150,  151. 


Mesohippus,  PL  47. 

"Method  of  Evolution,  The"  (Conn),  196, 

202. 

Mice,  Mendelian  phenomena,  46. 
Migration,  67. 

Mimicry,  aggressive,  148,  149;  conditions 
fulfilled  in,  148;  in  insects,  137,  139, 
PL  70,  PL  73,  PL  74,  PL  76,  PL  77; 
in  snakes,  145,  146,  PL  97;  Miillerian 
in  butterflies,  141,  PL  70,  PI.  76;  pro- 
tective, 138-148. 
Mind,  41;  development  of  human,  170; 

training  of  human,  181. 
Miohippus,  PL  47. 
Misumena  vatia,  PL  75. 
Milchella  re  pens,  159,  PI.  88. 
Mivart,  St.  George,  191. 
Modification,  inheritance  of  parental,  70-85, 

1 78  ct  scq. 

Monaxenia  darwinii,  104. 
Monkey,  ears,    168;  reject  noxious  insects, 

132;  relationship  to  man,  166. 
Moral  character,  effect  on  evolution,  191 ; 
growth   in   innate,    179;    improvement 
through  sexual  selection,  186;   training 
of,  180,  181. 
Morgan,  Lloyd,   29,  50,  51,  68,  132,  196, 

202. 

Morgan,  T.  H.,  60,  62,  202. 
Mormolycc  phylloides,  PL  62. 
Moss  insect,  124,  PL  61. 
Moths,  confusing  coloration,  150,  PL  83; 
elephant  hawk,  142,  143,  PL  78;  leaf, 
125,   PL  83;    mimicry,  139,  145,   146, 
PL  70,  PL  78;  protective  color,  PL  54, 
PL    55;     sexual    coloration,    PL    84; 
terrifying  attitude,  145;  warning  color, 
134,  PL  70;  waved-yellow,  125,  PL  63. 
Miiller,  Fritz,  136. 
Miillerian  mimicry  in  butterflies,  141,  PL 

70,  PL  76. 

Murray,  Sir  Robert,  5. 
Muscles,  of  human  ear,  96,   168,  PL  96; 
of  human  skin,  96;    vestigial  — of  tail 
in  man,  168,  PL  95. 

Mutation,    10-12,   39;    determinate,    193, 
195;    in  man,  182;    unit  qualities  aris- 
ing by  mutation  not  lost  or  modified  by 
cross-breeding,  43,  48. 
Mydas  clavatus,  PL  74. 
Mygnimia  amculus,  PL  73. 
Mysis   stage   in   development   of   lobster, 
PL  41;  stenolepis,  101,  PL  41. 


INDEX 


213 


Nascent  organs,  103. 

"  Natural  History  of  Plants,  The"  (Kerner), 
161,  202. 

Natural  selection,  3-50,  Intro,  xxiii;  man, 
171,172;  negative,  22;  sacrifices  indi- 
vidual for  welfare  of  race,  173. 

"Naturalist's  Wanderings  in  the  Eastern 
Archipelago,  A"  (Forbes),  129. 

"Natiirliche  Schopfungsgeschichte"  (Haeck- 
el),  PL  21. 

Nauplius,  187,  PI.  707. 

Nectar,  157,  158,  166. 

Negro,  segregation,  171. 

Nerice  bidentata,  PI.  56. 

Neritina  virginea,  var.  minor,  Frontis- 
piece, g. 

Nesocentor  milo,  PI.  25. 

New  Forest  —  segregation  of  sheep,  68. 

"New  Guinea,  Birds  of"  (Gould),  PL  25, 
PI.  26,  PL  80. 

New  Jersey  scrub  pine,  PL  87. 

Newton,  Intro,  xxi. 

Nictitating  membrane,  96,  169,  PL  36,  97. 

Nighthawk,  150,  PL  51,  PL  82. 

Nucleus,  controls  inheritance,  72. 

Nutrition  of  germ  cells,  71. 

Nymphalidce,  PL  77. 

Oahu  —  land  shells,  66 ;   map,  67. 
Objections  to  natural  selection,  33-50;    to 

sexual  selection,  59-63. 
Odor  of  warning-colored    butterflies,   134; 

of  flowers,  157. 

(Enothera  lamarckiana,  20,  193. 
Opfiibolus  doliatus,  PL  79. 
Opossum  —  geographical  distribution,  115. 
Orang,  166,  PL  97,  PL  94-96. 
Orchid  —  cross-fertilization,  159,  PL  89. 
Orchis  militaris,  PL  90. 
"Organic    Evolution,    The    Factors    of," 

(Cope),  202. 

Organic  selection,  29,  196. 
Orgya  antiqua,  PL  71. 
Origin  of  animals  from  plants,  3. 
"Origin  of  Civilization,  The"  (Lubbock), 

202. 

"Origin  of  Species,  The"  (Darwin),  Intro. 

xxii,  201. 

"Origin  of  the  Fittest,  The"  (Cope),  202. 
Oriole,     mimicry,     147,     PI-    8°>'     sexual 

coloration,  153. 
Oriolus  decipiens,  PL  80. 
Ornithoptera  (Papilio)  priamus,  153,  PL  84. 


Ornamentation,  extreme,  50. 

Orohippus,  PL  47. 

Orthogenesis,  42,  49-50,  63,  85,  86. 

Osborn,  H.  F.,  29,  50,  194,  196. 

Ostracoda,  188. 

Olaria,  PI.  36. 

Outdoor  life  and  sexual  selection,  184. 

Owl,  nictitating  membrane,  PL  36;  snowy, 

128. 
Oxyrrhopus  trigeminus,  PL  79. 

Pachyrhynchus  PL  73. 

Packard,  A.  S  ,  PI.  55,  PI.  56,  PI.  78. 

Paleontology,  89,  105-113. 

Palms,  fertilization,  156. 

Palndestrina  prolea,  PL  3. 

Palndina,  fossil  shells,  109,  no,  193,  195; 

orthogenesis,  49. 
Panolis  piniperda,  PL  56. 
Panthia  catnobita,  PL  55. 
Papilio  echerioides,  PL  76;    machaon,  PL 

71;    merope,    PL    76;     mimicry,    141; 

priamus,  PL  84;  ridleyanus,  PL  77. 
Papilionida,  134,  PL  76. 
Paradise,  bird  of,  sexual  coloration,  153. 
Paraguay,  segregation  of  wild  horses,  68. 
Paralichthys  dentata,  PL  48. 
Parasitism,  bee  parasites,  149;    effects  of, 

189;  sacculina,  187. 
Parental  care  and  length  of  life,  22. 
Parental  modifications,  inheritance  of,  70, 

71,  178. 

Parker,  T.  J.,  PI.  44- 
Parnassius  apollo,  PL  71. 
Parrot,  protective  color,  121. 
Partridge  berry,  159,  PL  88. 
Peacock,  sexual  coloration,  153. 
Peckham,  G.  W.  and  E.  G.,  spiders,  sexual 

selection    and    sexual    divergence,    58, 

PL  28,  PL  64;  wasps,  breeding  habits, 

80;  wasps,  color  sense,  164,  PI.  85. 
Pelagic  animals,  transparent,  119. 
Pelvis,  man  and  apes,  167,  PL  91. 
Perhybris  pyrr/ia,  PL  77. 
Permian  fossils,  108. 
Phanerogamia,  94. 
Pheasant,  Argus,  153,  PI-  24;    protective 

color,  120. 

"Pheasants"  (Tegetmeier),  PI.  24. 
Phenacodus,  in,  PL  46. 
Phidippiis  cardinalis,  PL  85. 
Philemon  plumegenis,  PL  80. 
Philohela  minor,  PL  50. 


214 


INDEX 


Phlogcenas  jobicitsis,  PL  25. 

Phoenicians,  178. 

P ftoras pis,  PI.  73. 

Phorodesmia  smargdaria,  PI.  55. 

Phyllium  siccifolium,Pl.  62. 

Ph'yllodcs  vertinellis,  PI.  83. 

Physics  and  life  processes,  191. 

Physiological  selection  and  segregation,  69, 

201. 

Phytolacca  decandra,  PI.  86. 
Pier  ids,  134,  PL  59. 

Pieris  brassiaf,  PL  56;  rapa,  color  adap- 
tation in  pupae,  123,  PL  59. 
Pigeon,  Phlogcenas,  PL  25;  rock,  33,  PL  20; 

varieties  of  domestic,  32,  PL  20. 
Pika,  protective  color,  PL  54. 
Pine,  156,  PL  87. 
Pinus  inops,  PL  87. 
"Plant  Breeding"  (De  Vries),  203. 
"Plant  Life"  (Coulter),  202,  203. 
Plasticity,   29,  30,   196;    of  man,  hinders 

evolution,  180. 

"Play  of  Animals,  The"  (Groos),  57. 
Plica  semilunaris,see  Nictitating  membrane. 
Pliocerous  clapsides  and  euryzonus,  PL  7 p. 
Pliohippus,  PL  47. 
Plover,  ring-necked,  PL  82. 
Polar-bear,  aggressive  color,  128. 
Polish  fowl,  skull,  32. 
Polisles,  breeding  habits,  80. 
Pollen,   food   of  insects,    157;    masses  of 

orchid,  159,  PL  89;   slow  to  sprout  on 

stigma  of  same  plant,  158;    tube,  755, 

156,  159- 

Pomatomus  saltatrix,  PL  48. 
Pompilus  alrox,  PL  74. 
Pond  snail,  development,  gg. 
Porgy,  change  of  color  in  grass,  123,  PL  58. 
Potato  beetle,  133,  PL  6g. 
Poulton,  E.  B.,  Preface  to  2d  Ed.,  x,  Intro. 

xxiii,  118,  142,  202,  PI.  59,  PI.  75. 
"Poultry,   New   Book   of"    (Wright),   32, 

PI.  16,  Pi.  18. 

"Prehistoric  Times"  (Lubbock),  202. 
Primates,  166,  169. 
Primulacea,  92. 
Principle,  general  principles  in  operation 

of  natural  selection,  22. 
Prionotus  cristatus,  PL  69. 
Progress  of  human  race,  176. 
Protective  coloration  and  resemblance,  118, 

II9-I27. 

Protoliippus,  PL  47. 


Providence,  Intro,  xxi. 

Pseudacrtea  boisduvalii,  PL  77. 

Psilura  monacha,  PL  56. 

Psyche  unicolor,  PL  56. 

Ptarmigan,    seasonal    color    change,    123, 

PL  57- 

Pterodaclylns  spectabilis,  PL  45. 

Plerogon  proserpina,  PL  70. 

Ptcrophryne  histrio,  126,  128,  PL  65. 

Public  opinion,  a  part  of  man's  environ- 
ment, 175,  182. 

Pupa,  color  adaptation,  123,  PL  59;  Logoa, 
125,  PL  63;  protective  color,  PL  56. 

Putoriiis  ermineus,  PL  67. 

Quail,  protective  color,  120,  PL  49. 

Rabbit,  confusing  coloration,  150;  em- 
bryos, PL  38;  illustration  of  mutation, 
21 ;  illustration  of  natural  selection,  17; 
protective  coloration  not  explicable  by 
the  inheritance  of  effects  of  use  and  dis- 
use, 78 ;  signal,  white  rump  patch,  149. 

Radiolaria,  skeletons,  PL  21. 

"  Recent  Progress  in  the  Study  of  Variation, 
Heredity  and  Evolution"  (Lock),  203. 

Recognition  marks,  149-150. 

Regeneration  and  inheritance  of  parental 
modification,  74. 

Reighard,  immunity  coloration,  137. 

Relationship,  key  to  classification,  0,3. 

"Relations  concerning  Ireland"  (Giraldus), 

3- 

Religion,  a  cause  of  segregation,  172. 

Reproduction,  among  unicellular  organ- 
isms, 71,  72;  and  length  of  life,  22,  24, 
25,  26;  asexual,  and  inheritance  of 
parental  modifications,  74 ;  birth-rate, 
13,  14;  easily  disturbed,  69;  effects  of 
destroying  organs  of,  36;  evolution 
centres  in,  85;  germ  cells  in  higher 
organisms,  72;  increase  in,  aids  in 
struggle  for  existence,  19;  organs  of, 
in  flowering  plants,  755,  156,  PL  86; 
regeneration  of  organs,  74. 

Ring-necked  plover,  recognition  marks, 
PL  82. 

Robin,  illustration  of  species,  91 ;  rate  of 
increase,  13;  sexual  coloration,  153. 

Robinson,  Louis,  PI.  102. 

Rock-rose,  755. 

Rocky  Mountains,  cause  of  segregation,  64. 

Romanes,  G.  J.,  19,  96,  107,  201,  203,  PI.  17, 


INDEX 


PI.   36-38,   PI.   43,   PL  44,   PL   75,   PI. 

93-99;    sexual   selection   in  birds,   56; 

physiological  selection,  69. 
RosacecE,  92. 

Rostellum  of  orchid,  159,  PI.  89. 
Roux,  Wilhelm,  196. 

Royal  Society  of  London,  goose  barnacle,  5. 
Ruffed  grouse,  PL  23. 
"Ruth,"  179. 

Sacculina,  187,  189,  PI.  101. 

Sacrum,  human,  with  tail  muscles,  PI.  95; 

of  man  and  apes,  167,  PL  91. 
Sail  is  pulex,  PL  28. 
Salamande/,    embryos,    PL    38;     tadpole, 

100;  warning  color,  135. 
Salamandra  maciilosa,  135. 
Salvia,  160;  glutinosa,  PL  88.- 
Sand-flounder,  PL  48. 
Sandpiper,  protective  color,  121. 
Sargassum  fish,  126,  128,  PL  65. 
Savage,  illustration  of  social  progress,  176 

el  seq. 

Savoy  cabbage,  PL  6,  PL  8. 
Scdoponis  undulatus,  PL  52. 
Scepasliis  pachyrhynchoides,  PL  73. 
Schistocerca,  6j. 
Sclater,  mimicry  of  leaf-cutting  ants,  140, 

148,  PL  75. 
Sea-horse,  126. 

Sea-lion,  nictitating  membrane,  PL  36. 
Seasonal  change  of  color,   123,   128,   131, 

PL  57,  PL  67. 
Secondary    sexual    characters,    see    Sexual 

selection ;  more  developed  in  female,  61. 
Seeds,  spiny,  not  evolved  through  inherit- 
ance of  the  effects  of  use,  79. 
Segregation,  63-70,  171 ;  see  also  44-50,  51- 
Selection,  artificial,  30-33;    germinal,  98; 

natural,  3-50;  organic,  29,  30,  180,  196  ; 

physiological,  69;  "selection  value,"  19, 

39- 

Selenia  letralunaria,  124. 

Sesia  culiciformis  and  tipuliformis,  PL  70. 

Seventeen-year  cicada,  54,  PL  29. 

Sexual  coloration,  152-154. 

Sexual  selection,  Intro,  xi,  50-63,  171,  172, 
J75,  176,  178,  181,  183-186;  a  cause 
of  segregation,  46 ;  objections  to,  59-63. 

Sheep,  Ancon,  segregation,  68;  Faroe 
Islands,  segregation,  68;  merinos  and 
heath  sheep  do  not  interbreed,  51 ; 
protective  color  of  wild,  122. 


Shore  birds,  protective  color,  121. 

Siberia,  former  warm  climate,  65. 

Signals  and  recognition  marks,  149-150. 

Silurian  fossils,  108. 

Simiidce,  167. 

Simplification  ("degeneration"),  187-190. 

Siphonophores,  transparency,  119. 

Sitana  minor,  PL  34. 

Skeletons    of    unicellular    organisms,    34, 

PL  21 ;  of  arm  of  vertebrates,  94. 
Skin  muscles  in  man,  96. 
Skunk,  135,  136,  PL  12. 
Skunk-cabbage,  158. 
Slavonia,  fossil  Paludina,  109. 
Small  pox,  174. 
Smelting  ore,  179. 

Smerinthus  tilice,  PL  55;  ocellata,  145. 
Snail,  development  of  pond,  99. 
Snails  of  Oahu,  segregation,  66. 
Snakes,  aggressive  color,  1 28 ;   behavior  of 

poisonous,    146;     hind   limbs,   95,   96; 

mimicry,  142,  PL  79;  protective  color, 

122. 

Snipe  —  protective  color,  121. 

Snow  grouse  —  seasonal  color  change,  123, 
PI-  57- 

Snowy -owl,  128. 

"Social  Direction  of  Human  Evolution" 
(Kellicott),  203. 

Socialism,  control  of  marriage,  185;  nature 
socialistic,  190. 

Social  progress,  an  end  in  itself,  179; 
vs.  evolution,  176-180. 

Soil,  relation  to  segregation,  66. 

Solea  concolor,  91,  92,  93. 

Soma,  distinguished  from  germ  cells,  73; 
relation  to  processes  of  reproduction,  77. 

Sparrow,  aggressive  color,  128;  protective 
color,  1 20,  PL  49. 

S  path  lira  solslitialis,  PL  26. 

Species,  incipient  sp.,  20,  34;  mean,  21; 
meaning  of,  90;  most  sp.  a  mixture  of 
races,  20;  preservation  of  species,  not 
of  individual,  secured  by  natural  selec- 
tion, 190;  use  of  word,  n. 

"Species  and  Varieties"  (De  Vries),  203. 

Spermophile,  protective  color,  PL  53. 

Sphinx  convolvuli,  PL  jj. 

Spiders,  aggressive  coloration,  128,  PI.  75; 
aggressive  mimicry  of  ants,  148;  ene- 
mies of,  123 ;  protective  coloration,  122, 
PL  75,  PL  85;  protective  resemblances, 
126,  PL  64;  resembling  bird  excre- 


2l6 


INDEX 


ment,  129;  sexual  coloration,  PL  85; 
sexual  selection,  53,  PL  28. 

Spilomyia  hamifera,  PI.  74. 

Spinal  column,  curvature  in  man  and  apes, 
169,  PL  99- 

Spizella,  pusilla,  PL  49. 

Sports,  12,  41,  48. 

Sprouts,  Brussels,  PL  7. 

Squirrel,  confusing  coloration,  151. 

Stability  of  certain  species,  9. 

Staghorn  beetle,  54,  PL  29. 

Starrish,  birth-rate,  15,  19. 

Stearns,  PI.  3. 

Sterility,  domestic  races  not  mutually 
sterile,  33 ;  of  crosses  between  certain 
individuals,  69;  of  crosses  between 
species,  33;  of  hybrids,  33,  43;  of 
soma  cells,  74 ;  of  worker  bees,  recently 
acquired,  80;  starting-point  in  forma- 
tion of  species,  34. 

Stick-like  insects,  124,  PL  61 ;  spider,  PL  64. 

Stridulating  organs,  54,  PL  29. 

Struggle  for  existence,  1 2-20 ;  between  near 
relatives,  27;  man,  172. 

Summary,  of  Part  I,  85  ;  of  color  in  animals, 
154- 

Supernaturalism,  xxi,  xxii. 

Superstition,  xxi. 

Survival  of  the  fittest,  17,  20. 

Swamping  of  varieties  by  cross-breeding, 
42-50. 

Swedish  turnip,  PL  7. 

Synageles  picata,  PL  28. 

Tadpole,  of  salamander,  100. 

Tail,  vestigial  muscles  in  man,  168,  PL  95. 

Taxonomy,  90. 

Teeth,  deterioration  of  human,  173;   man 

and  gorilla,  167,  PL  92. 
Tegetmeier,  PI.  14-17,  PL  18,  PL   19,  PL 

24. 
Tendency,  inherent,  in  evolution,  191 ;    in 

variation,  42. 

Terrifying  attitude,  142,  143,  145,  PL  78. 
Tetragnatha  grallalor,  122  ;  laboriosa,  PI.  85. 
"Thierleben"  (Brehm),  4,  24,  33,  135,  PL 

24,  PL  30,  PL  33,  PL  62,  PL  99. 
Thomisidce,  131. 
Thomson,  J.  A.,  193,  203. 
Thyroidopteryx  ephemeriformis,  23. 
Tiger,  aggressive  coloration,  131,  PL  68. 
Timor,  friar-bird  and  oriole,  146. 
Timor  Laut,  friar-bird  and  oriole,  146. 


Toad,  aggressive  coloration,  128,  135, 
PL  66;  rejects  noxious  insects,  132. 

Tody,  green,  protective  coloration,  121. 

Tortoise,  embryos,  PL  38. 

Trailing  arbutus,  odor,  157;   variation,  7. 

Transparency  of  pelagic  animals,  119. 

Tree-frogs,  protective  coloration,  127, PL  66. 

Tree-hopper,  mimicry  of  leaf-cutting  ants, 
140,  PL  75. 

Trends  in  evolution,  42,  49,  191,  193, 196. 

Trillium  grandiflornm,  PL  2. 

Triton    cristatus,    54,    PL    33;    punctatus, 

54- 

Trochilium  a  pi  forme,  PL  70. 

Tropidorhynchus,  147. 

Tunicates,  transparency  of  pelagic,  119. 

Turkey  cock,  52,  PL  27. 

Turnip,  31,  PL  9;  Swedish,  PL  7. 

Turtle,  embryos,  PL  38;  nictitating  mem- 
brane, PL  36. 

Twig-like  caterpillars,  124,  PI.  60. 

Typhlichthys,  97. 

Uloborus  plnmipes,  PL  64. 

Unicellular  organisms  and  inheritance  of 
parental  modifications,  71. 

Unit  characters,  43 ;  not  modified  in  inherit- 
ance by  cross-breeding,  43,  48. 

Unselfishness  in  marriage,  183. 

Ursus  mar  Him  us,  128. 

Use  and  disuse,  Intro,  xi;  effects  of,  71; 
inheritance  of  effects  of,  75,  76. 

"Utility"  (Romanes),  201. 

Utility,  and  segregation,  70;  uselessness 
of  certain  specific  characters,  35 ;  use- 
lessness of  organs  in  their  beginnings,  40. 

Vaccination,  175. 

Vanessa,  c-album,  PL  56;  to,  123;   urticce, 

PL  56,  PL  59. 
Variation,    7-12,    20,    42;     advantageous 

when  environment  changing,  29 ;  causes 

of,  82,  83,  84;    continuous,  10;   degree 

of  divergence,  9,  42  ;   determinate,  193 ; 

discontinuous,  10-12;    fluctuating,  10; 

in  Neritina,  9,  Frontispiece;    in  Palu- 

destrina,  PL  3;   in  trailing  arbutus,  7; 

in  Trillium,  PI.  2;    mutation,   10-12; 

stable,    10-12;     unequal    in    different 

species,  9 ;  unstable,  10. 
Varieties,  of  domestic  animals  and  plants, 

31 ;   of  horses,  30,  PL  4;   swamped  by 

intercrossing,  43. 


INDEX 


217 


Vermiform  appendix,  man  and  orang,  169, 

PI.  96. 
"Vertebrate  Embryology"  (Marshall),  PI. 

44- 
Vertebrates,  development  of,  99, 100,  PI.  38, 

PL  98;  varyingdegreesof  complexity,  97. 
Vespa,   breeding  habits,   80;    occidenlalis, 

PI.  74. 
Vestigial  structures,  95-98;    in  man,  166, 

PL  93-97- 
Vigcr,   correlated   with    secondary    sexual 

characters,  61. 
Vilmorin,  PL  87. 
Vinson,  PL  64. 

Viola  ciicullala,  90;  rostrata,  91. 
Violacea,  92. 
Voice,  in  bi'ds,  52. 
Volucellafacialis,  PI.  74. 
Vries,  de,  Hugo,  20,  21,  41,  193,  194,  203. 

Walking  stick,  124,  PL  61. 

Wallace,  A.  R.,  37,  50,  53.  61,  134,  146,  201, 

203,  PL  73- 

Wallihan,  A.  G.,  PL  81. 
W'arning  coloration,  131-136;  convergence, 

136,  PL  70,  PL  76. 

Warren,  E.  R.,  149,  PL  53,  PL  54,  PL  57. 
Wasps,  color  sense,  164;  enemies  of  spiders, 

123;     fertilizing   orchid,    160,   PL   89; 


mimicked  by  other  insects,  139,  PL  70, 

PI-  73 >'   protected  by  stings,  132,  137; 

warning  color,  132,  PI.  73,  PL  74. 
Waved-yellow  moth,  125,  PL  63. 
Weasel,  PL  67. 
Weismann,    August,    acquired   characters, 

70;    germinal  selection,  98;    also,  124, 

144,  201,  203,  PL  76,  PL  77,  PL  101. 
Westermarck,  202. 
Whale,  hind  limbs,  95,  96. 
Wheel,  invention  of,  179. 
White  Mountains,  flora,  114. 
"Wild    Flowers   of    America"    (Goodale), 

PL  88. 
Wing,  of  bird  and  of  butterfly,  94;    fore 

limbs  of  vertebrates,  94. 
Wolf,  aggressive  coloration,  128. 
Woodcock,  protective  coloration,  52,  121, 

PL  50. 
Wright,  Lewis,  32,  PL  16,  PL  18. 

Xiphophorus  hellerii,  PL  32. 

Yellow-jacket,    warning    color,    132,    138, 
PI-  74- 

Zenzera  asculi,  PL  70. 
Zittel,  K.,  PL  44,  PI-  45- 
Zygaena,  PL  56,  PL  70. 


COLUMBIA  UNIVERSITY  BIOLOGICAL  SERIES. 

DESIGNED  FOR  INDEPENDENT  READING  AND  AS  TEXT-BOOKS 

FOR  LECTURE  AND  LABORATORY  COURSES 

OF  INSTRUCTION. 

EDITED    BY 

HENRY   FAIRFIELD   OSBORN,  Sc.D.,  LL.D., 

De  Costa  Professor  of  Zoology,  Columbia  University, 

AND 

EDMUND   B.  WILSON,  Ph.D.,  LL.D., 

Professor  of  Zoology,  Columbia  University. 


VOL.  L    FROM  THE  GREEKS  TO  DARWIN. 

THE  DEVELOPMENT  OF   THE   EVOLUTION  IDEA. 

By  HENRY  FAIRFIELD  OSBORN,  Sc.D.,  LL.D. 
Cloth.    8vo.    259  pages.    Illustrated.    Price,  $2.00  net. 


OPINIONS  OF  THE  PRESS. 


"  Professor  Henry  Fairfield  Osborn  has  rendered 
an  important  service  by  the  preparation  of  a  concise 
history  of  the  growth  of  the  idea  of  Evolution.  The 
chief  contributions  of  the  different  thinkers  from 
Thales  to  Darwin  are  brought  into  clear  perspective, 
and  a  just  estimate  of  the  methods  and  results  of  each 
one  is  reached.  The  work  is  extremely  well  done, 
and  it  has  an  added  value  of  great  importance  in  the 
fact  that  the  author  is  a  trained  biologist.  Dr.  Os- 
born is  himself  one  of  the  authorities  in  the  science 
of  Evolution,  to  which  he  has  made  important  con- 
tributions. He  is  therefore  in  a  position  to  estimate 
the  value  of  scientific  theories  more  justly  than  would 
be  possible  to  one  who  approached  the  subject  from 
the  standpoint  of  metaphysics  or  that  of  literature." 
—  President  DAVID  STARR  JORDAN, 

in  The  Dial,  Chicago. 

"  A  somewhat  new  and  very  interesting  field  of  in- 
quiry is  opened  in  this  work,  which  is  devoted  to 
demonstrating  that  the  doctrine  of  Evolution,  far 
from  being  a  child  of  the  middle  of  the  nineteenth 
century,  of  sudden  birth  and  phenomenally  rapid 
growth,  as  it  is  by  many  supposed  to  be,  has  really 
been  in  men's  minds  for  ages.  It  appears  in  the 
germ  in  the  earliest  Greek  philosophy;  in  vigorous 
childhood  in  the  works  of  Aristotle  ;  in  adolescence 
at  the  closing  period  of  the  last  century ;  and  reaches 
full-grown  manhood  in  our  own  age  of  scientific 
thought  and  indefatigable  research." 

—  New  Science  Review. 

"  This  is  a  timely  book.  For  it  is  time  that  both 
the  special  student  and  general  public  should  know 
that  the  doctrine  of  Evolution  has  cropped  out  of  the 
surface  of  human  thought  from  the  period  of  the 
Greek  philosophers,  and  that  it  did  not  originate 
with  Darwin,  and  that  natural  selection  is  not  a 
synonym  of  Evolution.  .  .  .  The  book  should  be 


widely  read,  not  only  by  science  teachers,  by  biologi- 
cal students,  but  we  hope  that  historians,  students  of 
social  science,  and  theologians  will  acquaint  them- 
selves with  this  clear,  candid,  and  catholic  statement 
of  the  origin  and  early  history  of  a  theory,  which  not 
only  explains  the  origin  of  life-forms,  but  has  trans- 
formed the  methods  of  the  historian,  placed  philoso- 
phy on  a  higher  plane,  and  immeasurably  widened 
our  views  of  nature  and  of  the  Infinite  Power  work- 
ing in  and  through  the  universe." 

-  Professor  A.  S.  PACKARD, 

in  Science,  New  York. 

"  This  is  an  attempt  to  determine  the  history  of 
Evolution,  its  development  and  that  of  its  elements, 
and  the  indebtedness  of  modern  to  earlier  investi- 
gators. The  book  is  a  valuable  contribution;  it  will 
do  a  great  deal  of  good  in  disseminating  more  accu- 
rate ideas  of  the  accomplishments  of  the  present  as 
compared  with  the  past,  and  in  broadening  the  views 
of  such  as  "  -ve  confined  themselves  too  closely  to 
the  recent  or  to  specialties.  ...  As  a  whole  the 
book  is  admirable.  The  author  has  been  more  im- 
partial than  any  of  those  who  have  in  part  anticipated 
him  in  the  same  line  of  work."  —  The  Nation. 

"  But  whether  the  thread  be  broken  or  continuous, 
the  history  of  thought  upon  this  all-important  subject 
is  of  the  deepest  interest,  and  Professor  Osborn's 
work  will  be  welcomed  by  all  who  take  an  intelligent 
interest  in  Evolution.  Up  to  the  present,  the  pre- 
Darwinian  evolutionists  have  been  for  the  most  part 
considered  singly,  the  claims  of  particular  naturalists 
being  urged  often  with  too  warm  ~n  enthusiasm. 
Professor  Osborn  has  undertaken  a  more  compre- 
hensive work,  and  with  well-balanced  judgment 
assigns  a  place  to  each  writer." 

—  Professor  EDWARD  B.  POULTON, 

in  Nature,  London. 


FIRST   EDITION   PUBLISHED   OCTOBER,    1894. 

VOL.  H.  AMPHIOXUS  AND  THE  ANCESTRY  OF 
THE  VERTEBRATES. 

By  ARTHUR  WILLEY,  Sc.D.,  Balfonr  Student  of  the  University  of  Cambridge. 
316  pages.     135  Illustrations.    Price,  $2.50  net. 

"  This  important  monograph  will  be  welcomed  by 
all  students  of  zoology  as  a  valuable  accession  to  the 
literature  of  the  theory  of  descent.  More  than  this, 
the  volume  bears  internal  evidence  throughout  of 
painstaking  care  in  bringing  together,  in  exceedingly 
readable  form,  all  the  essential  details  of  the  structure 
and  metamorphosis  of  Amphioxus  as  worked  out  by 
anatomists  and  embryologists  since  the  time  of  Pallas, 
its  discoverer.  The  interesting  history  of  the  changes 
it  undergoes  during  metamorphosis,  especially  its  sin- 
gular symmetry,  is  clearly  described  and  ingenious 
explanations  of  the  phenamena  are  suggested.  Most 
important,  perhaps,  are  the  carefully  suegested  homol- 
ogies  of  the  organs  of  A mphioxus  with  those  ot  the 
embryos  of  the  Vertebrates  above  it  in  rank,  especially 
those  of  the  Marsipobranchs  and  Selachians.  Though 
the  comparisons  with  the  organisms  next  below  Am- 
phioxus, such  as  Ascidians,  Balanoglnssus.  Cepha- 
lodiscus,  Rhabdopleura,  and  the  Echinoderms, 
will  be  found  no  less  interesting.  In  short,  the  book 
may  be  commended  to  students  already  somewhat 
familiar  with  zoological  facts  and  principles,  as  an 
important  one  to  read.  They  may  thus  be  brought 
to  appreciate  to  what  an  extent  the  theory  of  des 
is  indebtec 


ed  to  the  patient  labors  of  the  zoologists  of 


the  last  forty  years  for  a  secure  foundation  in  observed 
facts,  seen  in  their  correlations,  according  to  the  com- 
parative method.  .  .  .  The  present  work  contains 
everything  that  should  be  known  about  Amphioxus, 
besides  a  great  deal  that  is  advantageous  to  know 
about  the  Tunica ta,  Balatwglossus,  and  some  other 
types  which  come  into  structural  relations  with  Am- 
phioxus." 

—  Professor  JOHN  A.  RYDER, 

in  The  American  Naturalist,  Philadelphia. 

"  The  observations  on  Amphioxus  made  before  the 
second  half  of  the  present  century,  amongst  which 
those  of  Johannes  M  iiller  take  a  foremost  place,  showed 
that  this  remarkable  animal  bears  certain  resemblances 
to  Vertebrates ;  and  since  then  its  interest  in  this  re- 
spect has  gradually  become  more  apparent.  ...  A 
consecutive  history  of  the  more  recent  observations 
was,  therefore,  greatly  needed  by  those  whose  oppor- 
tunities did  not  permit  them  to  follow  out  the  matter 
for  themselves,  and  who  will  welcome  a  book  written 
in  an  extremely  lucid  style  by  a  naturalist  who  can 
speak  with  authority  on  the  subject." 

—  Professor  W.  NEWTON  PARKER, 

in  Nature,  London. 


VOL.  m.    FISHES,  LIVING  AND  FOSSIL. 

AN  INTRODUCTORY  STUDY. 

By  BASHFORD  DEAN,  Ph.D.,  Adjunct  Professor  of  Zoology,  Columbia  University. 
300  pages.    344  Illustrations.    Price,  $2.50  net. 

This  work  has  been  prepared  to  meet  the  need  of  the  general  student  for  a  concise  knowledge  of  the  living 
and  extinct  Fishes  It  covers  the  recent  advances  in  the  comparative  anatomy,  embryology,  and  palaeontology 
of  the  five  larger  groups  of  Lampreys,  Sharks,  Chimaeroids,  Teleostomes,  and  Dipnoans  —  the  aim  being  to 
furnish  a  well-marked  ground  plan  of  Ichthyology  The  figures  are  mainly  original  and  designed  to  aid  in  prac- 
tical work  as  well  as  to  illustrate  the  contrasts  in  the  development  of  the  principal  organs  through  the  five  groups. 

work.  The  suggestions  here  offered  may  be  of  use 
for  another  edition.  That  another  may  be  called  for, 
we  may  hope.  For  the  work  as  it  is,  and  for  the  care 
and  thought  bestowed  on  it,  our  thanks  are  due." 

—  TH  EODO  K  E  G !  LL, 

in  Science,  New  York. 

"  L'ouvrage  de  M.  Bashford  Dean  nous  parait  fait 
avec  soin:  les  illustrations  sont  excellentes  et  tres 
nombreuses,  et  il  me'rite  le  meilleur  accueil  de  la  part 
des  zoologistes." 

—  CH.  BRONGNIART, 

in  Le  Revue  Scientifiqne,  Paris. 


"The  intense  specialization  which  prevails  in 
zoology  at  the  present  day  can  lead  to  no  other  result 
than  this,  that  a  well-educated  zoologist  who  becomes 
a  student  of  one  group  is  in  a  few  years  quite  lelt 
behind  by  the  student  oT  other  groups.  Books, 
therefore,  like  those  of  Mr.  Dean  are  necessary  for 
zoologists  at  large." 

—  The  Athenceum,  London. 

"  Dr.  Bashford  Dean  is  known  to  zoologists,  first, 
as  the  author  of  exhaustive  and  critical  articles  in  the 
publications  of  the  United  States  Fish  Commission, 
on  the  systems  of  oyster  culture  pursued  in  Europe, 
and,  secondly,  as  an  embryologist  who  has  lately  been 
doing  good  work  on  the  development  of  various  Ga- 
noid fishes  and  the  comparison  that  may  be  instituted 
with  Teleostei.  His  recent  addition  to  the  well-known 
'  Columbia  University  Biological  Series,'  now  being 
brought  out  by  The  Macmillan  Company,  under  the 
editorship  of  Professor  H.  F.  Osborn,  is  an  interesting 
volume  upon  fishes,  in  which  considerable  prominence 
is  given  to  the  fossil  forms,  and  the  whole  subject  is 
presented  to  us  from  the  point  of  view  of  the  evolu- 
tionist. This  is  the  characteristic  feature  of  the  book. 
From  the  very  first  page  of  the  introduction  to  the 
last  page  in  the  volume,  preceding  the  index,  which 
is  a  table  of  the  supposed  descent  of  the  groups  of 
fishes,  the  book  is  full  of  the  spirit  and  the  language 
of  evolution."  —  Professor  W.  A.  HERDMAN, 

in  Nature,  London. 

"  The  length  to  which  this  review  has  extended 
must  be  evidence  of  the  importance  of  Dr.  Dean's 


the  history  of  Ichthyology, 
and- 


" For  the  first  time 

students  are  now  provided  with  an  elementary  han 
book  affording  a  general  view  of  the  whole  subject.  .  .  . 
The  last  sixty  pages  of  the  volume  are  devoted  to 
a  list  of  derivations  of  proper  names,  a  copious  bibli- 
ography, and  a  series  of  illustrated  tabular  statements 
of  the  anatomical  characters  of  the  great  groups  of 
fshes.  These  sections  bear  signs  of  having  been 
prepared  most  carefully  and  laboriously,  and  form  an 
admirable  appendix  for  purposes  of  reference.  There 
will  be  much  difference  of  opinion  among  specialists 
as  to  the  value  of  some  of  the  tables  and  the  judgment 
pronounced  by  the  author;  but  we  have  detected  a 
very  small  proportion  of  errors  for  so  bold  an  enter- 
prise, and  students  of  the  lower  VerteKrata  are  much 
indebted  to  Dr.  Dean  for  an  invaluable  compendium." 
—  ARTHUR  SMITH  WOODWARD, 

in  Natural  Science,  London. 


VOL.  IV.    THE  GELL  IN  DEVELOPMENT  AND 
INHERITANCE. 

By  EDMUND   B.  WILSON,  Ph.D.,  LL.D.,      •    . 

Professor  of  Zoology,  Columbia  University. 

371  pages.     142  Illustrations.     Price,  $3.50  net. 


"  In  the  highest  degree  fascinating.  ...  It  is  a 
matter  for  congratulation  that  Professor  Wilson  has 
given  us  in  our  own  speech  a  book  which  is  second 
to  none  in  the  clear  and  comprehensive  manner  in 
which  the  facts  of  cell-structure  and  division  are  set 
forth,  and  the  masterly  way  in  which  the  principal 
theories  are  stated  and  criticised."  — Nature. 

"  It  certainly  takes  rank  at  once  among  the  most 
important  biological  works  of  the  period." 

—  Science. 

"  We  heartily  recommend  this  book.  There  are 
many  practitioners  who  have  neither  time  nor  disposi- 


tion to  read  the  larger  treatises  on  botany  or  histology 
in  which  the  modern  views  on  the  structure  and  func- 
tions of  the  cell  are  to  be  found  in  detail.  ...  lr, 
the  present  volume  they  will  find  an  admirable  expo- 
sition of  the  knowledge  that  has  been  acquired  during 
the  last  twenty  years."  —  London  Lancet. 

"  One  of  the  very  best  scientific  manuals  published 
in  America.  ...  A  noteworthy  characteristic  of  the 
book  is  its  thoroughness.  .  .  .  Students  and  inves- 
tigators of  biology,  in  whatever  department  they  may 
be  working,  ought  to  be  familiar  with  this  important 
work."  —  New  York  Nation. 


VOL.  V.    THE  FOUNDATIONS  OF  ZOOLOGY. 

By  WILLIAM  KEITH  BROOKS, 

Professor  of  Zoology,  Jo/ins  Hopkins   University. 
8vo.     Cloth,     viii  +  339  pages.     Price,  $2.50  net. 


"  A  book  that  will  live  as  a  permanent  addition  to 
the  common  sense  of  science.  It  belongs  to  literature 
as  well  as  to  science.  It  belongs  to  philosophy  as 


much  as  to  either,  for  it  is  full  of  that  fundamental 
wisdom  about  realities  which  alone  is  worthy  of  the 
name  of  philosophy."  — Science 


The  object  of  thi 
exh 


VOL.  VI.    THE  PROTOZOA. 

By  GARY  N.  CALKINS,  Ph.D., 

Instructor  in  Zoology,  Columbia   University 

8vo.     Cloth.     365  pages.     Price,  $3.00  net. 

volume  is  to  set  forth  the  main  characte 


ive  description.     It  is  intended  for  students  and  for  general  r 
re,  and  what  their  relations  are  to  current  biological  problems. 


f  the  Protozoa  without  undertaking  an 
:aders  who  wish  to  know  what  the  Pro- 
In  the  first  few  chapters  of  the  book 

the  Protozoa  are  treated  as  a  phylum  of  the  animal  kingdom.  A  short  historical  sketch  leading  up  to  the 
present  systems  of  classification  is  followed  by  a  general  description  of  the  group,  touching  upon  some  of  the 
more  special  subjects,  such  as  mode  of  life,  motion,  excretion,  respiration,  reproduction,  colony-formation, 
encystmem,  etc  ,  and  this  is  followed  by  more  general  subjects  dealing  with  the  Protozoa  in  relation  to  man 
and  other  animals;  eg  their  sanitary  aspects,  parasitism,  symbiosis,  etc. 

In  the  final  chapter  the  Protozoa  are  dealt  with  from  the  standpoint  of  phylogeny.  Theories  as  to  the 
origin  of  life,  spontaneous  generation,  and  the  relations  of  the  classes  of  Protozoa  to  one  another  are  con- 
sidered, and  the  volume  ends  with  a  discussion  of  the  various  views  regarding  the  origin  of  the  Metazoa  from 
the  Protozoa. 

VOL.  VH.    REGENERATION. 

By  THOMAS   HUNT  MORGAN, 

Professor  of  Biology,  Bryn  Mawr  College, 
Author  of   "THE  DEVELOPMENT  OF  THE  FROG'S  EGG." 


VOL.  Vm.    AN  INTRODUCTION  TO  COMPARA- 
TIVE  NEUROLOGY. 

By  OLIVER   S.   STRONG,  Ph.D. 
THE   MACMILLAN  COMPANY,  66  Fifth  Avenue,  New  York. 


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